netdev: fix drivers/net/phy/ kernel-doc warnings

Fix kernel-doc warnings in drivers/net/phy:

Warning(drivers/net/phy/mdio_bus.c:109): No description found for parameter 'mdio_bus_np'
Warning(drivers/net/phy/mdio_bus.c:109): Excess function parameter 'mdio_np' description in 'of_mdio_find_bus'

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: David S. Miller <davem@davemloft.net>