etherdevice: Use ether_addr_copy to copy an Ethernet address

Some systems can use the normally known u16 alignment of
Ethernet addresses to save some code/text bytes and cycles.

This does not change currently emitted code on x86 by gcc 4.8.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: David S. Miller <davem@davemloft.net>