etherdevice: Optimize a few is_<foo>_ether_addr functions

If CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS is set,
several is_<foo>_ether_addr functions can be slightly
improved by using u32 dereferences.

I believe all current uses of is_zero_ether_addr and
is_broadcast_ether_addr are u16 aligned, so always use
u16 references to improve those functions performance.

Document the u16 alignment requirements.

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