bridge: allow setting hash_max value

bridge: allow setting hash_max value

If the number of entries in the MDB exceeds hash_max then the
multicast snooping capabilities of the bridge are disabled
automatically.

The default value for hash_max is 512 which is already exceeded by some
wireless community mesh networks. They need to be able to set a higher
value.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>