mlxsw: spectrum_router: Allocate FID according to PVID

For bridge netdevices, instead of assuming that the router traffic is on
VLAN 1, look at the bridge PVID.

This patch assumes that the PVID doesn't change after the router
interface is created (i.e. after the IP address is assigned).

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