net/macb: check all address registers sets

The macb driver in u-boot uses the first register set while
the at91_ether driver in u-boot uses the second register set.

By checking all register set, like at91_ether does, this code
can be shared between the drivers.

This only changes behavior on macb if no vaild address
is found in the first register set.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>