Replace homegrown mac addr checks with faster defs from etherdevice.h

Note that this will also prevent any rlb arp updates for multicast
addresses, however this should have been forbidden anyway.
Signed-off-by: NDebabrata Banerjee <dbanerje@akamai.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>