net/at91_ether: use ethtool and mdio from macb

This rips out the at91_ether phy handling and ethtool stuff
and replace it with equivalent stuff from macb.

The only thing lost is the phy irq support from at91_ether,
but this can be added to macb and then benefit all users.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>