mlxsw: spectrum: Change bridge port attributes only when bridged

Bridge port attributes are offloaded to hardware when invoked with SELF
flag set, but it really makes no sense to reflect them when port is not
bridged.

Allow a user to change these attribute only when port is bridged and
initialize them correctly when joining or leaving a bridge.

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>