bridge: mcast: Use correct define in MDB dump

'MDB_PG_FLAGS_PERMANENT' and 'MDB_PERMANENT' happen to have the same
value, but the latter is uAPI and cannot change, so use it when dumping
an MDB entry.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>