ltq-deu: fix 6.12 kernel build warnings on lantiq/xway

ltq-deu: fix 6.12 kernel build warnings on lantiq/xway

This patch fixes the following build warnings by adding
missing prototypes:

/home/db/owrt/build_dir/target-mips_24kc_musl/linux-lantiq_xway/ltq-deu-danube/ltq-deu/ifxmips_deu_danube.c:100:5: error: no previous prototype for 'input_swap' [-Werror=missing-prototypes]
  100 | u32 input_swap(u32 input)
      |     ^~~~~~~~~~
/home/db/owrt/build_dir/target-mips_24kc_musl/linux-lantiq_xway/ltq-deu-ar9/ltq-deu/ifxmips_deu_ar9.c:91:5: error: no previous prototype for 'input_swap' [-Werror=missing-prototypes]
   91 | u32 input_swap(u32 input)
      |     ^~~~~~~~~~
/home/db/owrt/build_dir/target-mips_24kc_musl/linux-lantiq_xway/ltq-deu-ar9/ltq-deu/ifxmips_deu_ar9.c:131:6: error: no previous prototype for 'chip_version' [-Werror=missing-prototypes]
  131 | void chip_version(void)
      |      ^~~~~~~~~~~~

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Link: https://github.com/openwrt/openwrt/pull/18744
Signed-off-by: Robert Marko <robimarko@gmail.com>