ar71xx: ag71xx_phy: Fix compilation for debug messages

NB: Error only appears with ag71xx debug messages and dynamic printk
enabled.  This is probably why no one has caught it before.

Previously phy probe debug messages used old (now wrong) functions
to get the phy name for printing.  There was also the chance of
a NULL pointer in the event no phy_device was found.

Signed-off-by: Daniel F. Dickinson <cshored@thecshore.com>