enic: minimize pkt filter updates to firmware

In set_multicast(), only push pkt filter changes down to firmware if
pkt filter actually changes.

Signed-off-by: Vasanthy Kolluri <vkolluri@cisco.com>
Signed-off-by: Scott Feldman <scofeldm@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>