net/macb: support reversed hw addr

This is used on one AT91RM9200 board where a bootloader stores
the Ethernet address in the wrong order.

Support this on macb so address setting functions can be shared
with the at91_ether driver.

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