mlxsw: spectrum: Always decrement bridge's ref count

Since we only support one VLAN filtering bridge we need to associate a
reference count with it, so that when the last port netdev leaves it, we
would know that a different bridge can be offloaded to hardware.

When a LAG device is memeber in a bridge and port netdevs are leaving
the LAG, we should always decrement the bridge's reference count, as it's
incremented for any port in the LAG.

Fixes: 4dc236c3

 ("mlxsw: spectrum: Handle port leaving LAG while bridged")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>