[NET]: Add is_multicast_ether_addr() in include/linux/etherdevice.h

This patch adds is_multicast_ether_addr() to go along with
is_valid_ether_addr() and friends. It then changes
is_valid_ether_addr() to use the new macro, and fixes up the comment
on that function to move implementation details out of the API doco.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: David S. Miller <davem@davemloft.net>