bridge: remove redundant check on err in br_multicast_ipv4_rcv

br_ip4_multicast_mrd_rcv only return 0 and -ENOMSG,
no other negative value

Signed-off-by: Li RongQing <lirongqing@baidu.com>
Acked-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>