net: bridge: explicitly convert between mdb entry state and port group flags

When creating a new multicast port group, there is implicit conversion
between the __u8 state member of struct br_mdb_entry and the unsigned
char flags member of struct net_bridge_port_group. This implicit
conversion relies on the fact that MDB_PERMANENT is equal to
MDB_PG_FLAGS_PERMANENT.

Let's be more explicit and convert the state to flags manually.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20201028234815.613226-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>