net: bridge: remove unneeded variable 'err'

function br_multicast_toggle now always return 0,
so the variable 'err' is unneeded.
Also cleanup dead branch in br_changelink.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>