bridge: mcast: Default back to multicast enabled state

Commit 13cefad2 ("net: bridge: convert and rename mcast disabled")
converted the 'multicast_disabled' field to an option bit named
'BROPT_MULTICAST_ENABLED'.

While the old field was implicitly initialized to 0, the new field is
not initialized, resulting in the bridge defaulting to multicast
disabled state and breaking existing applications.

Fix this by explicitly initializing the option.

Fixes: 13cefad2 ("net: bridge: convert and rename mcast disabled")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>