net: bridge: Extend br_vlan_get_pvid() for bridge ports

Currently, the function only works for the bridge device itself, but
subsequent patches will need to be able to query the PVID of a given
bridge port, so extend the function.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>