net/ucc_geth: allow to set mac address on running device

Inspired by the patch for 8139too (bda6a15a).

Currently we can't set mac address on a running ucc_geth device.
But this is needed when you use this device as a bonding slave in
bonding device in balance-alb mode. So add this feature for ucc_geth
device.

Signed-off-by: Kevin Hao <kexin.hao@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>