net: split eth_mac_addr for better error handling

When we set mac address, software mac address in system and hardware mac
address all need to be updated. Current eth_mac_addr() doesn't allow
callers to implement error handling nicely.

This patch split eth_mac_addr() to prepare part and real commit part,
then we can prepare first, and try to change hardware address, then do
the real commit if hardware address is set successfully.

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Amos Kong <akong@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>