Some Ethernet switches might only be able to support disabling multicast
snooping globally, which is an issue for example when several bridges
span the same physical device and request contradictory settings.

Propagate the return value of br_mc_disabled_update() such that this
limitation is transmitted correctly to user-space.
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>