net/macb: clear unused address register

Only the first register set is used for matching but
we support getting the initial hw addr from any of
the registers.

To prevent stale entries and false matches clear unused
register sets. This most important for the at91_ether
driver where u-boot always uses the 2nd register set.

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