etherdevice: Add ether_addr_equal_unaligned

Add a generic routine to test if possibly unaligned
to u16 Ethernet addresses are equal.

If CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS is set,
this uses the slightly faster generic routine
ether_addr_equal, otherwise this uses memcmp.

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