smsc911x: allow setting of mac address

This patch replaces the generic eth_mac_addr function with one that
also updates the hardware mac address registers.  It also renames the
existing smsc911x_set_mac_address function to
smsc911x_hw_set_mac_address for clarity.

Newer LAN911x and all LAN921x devices also support changing the mac
address while the device is running, which is useful for some bonding
modes.

Signed-off-by: Steve Glendinning <steve.glendinning@smsc.com>
Signed-off-by: David S. Miller <davem@davemloft.net>