Set mc-promisc (multicast promiscuous) mode on an interface, only if it is
*not already* in that mode.

Also removed logs that report interface being set to multicast
promiscous mode. In an earlier comment on the netdev list such log messages
were deemed unnecessary as this behaviour is common across most of the
ethernet drivers.

Signed-off-by: Kalesh AP <kalesh.purayil@emulex.com>
Signed-off-by: Sathya Perla <sathya.perla@emulex.com>
Signed-off-by: David S. Miller <davem@davemloft.net>