ar71xx: fix compiler warnings in mdio-bitbang.c for 4.4

ar71xx: fix compiler warnings in mdio-bitbang.c for 4.4

Building Linux 4.4 for ar71xx throws the following warnings in
drivers/net/phy/mdio-bitbang.c:

    CC      drivers/net/phy/mdio-bitbang.o
  In file included from include/linux/irqflags.h:14:0,
                   from drivers/net/phy/mdio-bitbang.c:20:
  drivers/net/phy/mdio-bitbang.c: In function 'mdiobb_read':
  include/linux/typecheck.h:11:18: warning: comparison of distinct pointer types lacks a cast
    (void)(&__dummy == &__dummy2); \
                    ^
  include/linux/irqflags.h:63:3: note: in expansion of macro 'typecheck'
     typecheck(unsigned long, flags); \
     ^
  include/linux/irqflags.h:124:3: note: in expansion of macro 'raw_local_irq_save'
     raw_local_irq_save(flags);   \
     ^
  drivers/net/phy/mdio-bitbang.c:162:2: note: in expansion of macro 'local_irq_save'
    local_irq_save(flags);
    ^
  include/linux/typecheck.h:11:18: warning: comparison of distinct pointer types lacks a cast
    (void)(&__dummy == &__dummy2); \
                    ^
  include/linux/irqflags.h:68:3: note: in expansion of macro 'typecheck'
     typecheck(unsigned long, flags); \
     ^
  include/linux/irqflags.h:126:39: note: in expansion of macro 'raw_local_irq_restore'
   #define local_irq_restore(flags) do { raw_local_irq_restore(flags); } while (0)
                                         ^
  drivers/net/phy/mdio-bitbang.c:175:2: note: in expansion of macro 'local_irq_restore'
    local_irq_restore(flags);
    ^
  drivers/net/phy/mdio-bitbang.c:159:11: warning: unused variable 'i' [-Wunused-variable]
    int ret, i;
             ^
  In file included from include/linux/irqflags.h:14:0,
                   from drivers/net/phy/mdio-bitbang.c:20:
  drivers/net/phy/mdio-bitbang.c: In function 'mdiobb_write':
  include/linux/typecheck.h:11:18: warning: comparison of distinct pointer types lacks a cast
    (void)(&__dummy == &__dummy2); \
                    ^
  include/linux/irqflags.h:63:3: note: in expansion of macro 'typecheck'
     typecheck(unsigned long, flags); \
     ^
  include/linux/irqflags.h:124:3: note: in expansion of macro 'raw_local_irq_save'
     raw_local_irq_save(flags);   \
     ^
  drivers/net/phy/mdio-bitbang.c:185:2: note: in expansion of macro 'local_irq_save'
    local_irq_save(flags);
    ^
  include/linux/typecheck.h:11:18: warning: comparison of distinct pointer types lacks a cast
    (void)(&__dummy == &__dummy2); \
                    ^
  include/linux/irqflags.h:68:3: note: in expansion of macro 'typecheck'
     typecheck(unsigned long, flags); \
     ^
  include/linux/irqflags.h:126:39: note: in expansion of macro 'raw_local_irq_restore'
   #define local_irq_restore(flags) do { raw_local_irq_restore(flags); } while (0)
                                         ^
  drivers/net/phy/mdio-bitbang.c:200:2: note: in expansion of macro 'local_irq_restore'
    local_irq_restore(flags);
    ^

These are caused by the 900-mdio_bitbang_ignore_ta_value and
901-phy-mdio-bitbang-prevent-rescheduling-during-command patches.

The first patch removes some code but it does not remove the variable
which is used by the removed code only.

The second patch adds local_irq_{save,restore} calls. The type of the
argument of these calls must be 'unsigned long', but the patch defines
the variable as 'long'.

Fix both patches to silence the warnings.

Signed-off-by: Gabor Juhos <juhosg@freemail.hu>