bridge: call br_netpoll_disable in br_add_if

When netdev_set_master faild in br_add_if, we should
call br_netpoll_disable to do some cleanup jobs,such
as free the memory of struct netpoll which allocated
in br_netpoll_enable.

Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>
Acked-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>