net: bridge: add the multicast_flood flag attribute to brport_attrs

When I added the multicast flood control flag, I also added an attribute
for it for sysfs similar to other flags, but I forgot to add it to
brport_attrs.

Fixes: b6cb5ac8

 ("net: bridge: add per-port multicast flood flag")
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>