ath25: 4.9: fix Ethernet tiny issues

Few tiny fixes for issues caused by changes in the upstream:
 - do not touch PHY IRQ array (core code initializes it itself now)
 - add missed SET_NETDEV_DEV() invocation (causes segfault during phy
   connection)
 - use phy API inside the MDIO probe function instead of direct field
   access (consider phy structure changes in upstream and prevent
   similar issues in the future)

Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>