rose_dev: fix memcpy-bug in rose_set_mac_address

If both addresses equal, nothing needs to be done. If the device is down,
then we simply copy the new address to dev->dev_addr. If the device is up,
then we add another loopback device with the new address, and if that does
not fail, we remove the loopback device with the old address. And only
then, we update the dev->dev_addr.

Signed-off-by: Daniel Borkmann <daniel.borkmann@tik.ee.ethz.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>