mlxsw: spectrum_switchdev: Prepare function for VLAN-aware bridges

The vxlan_join() function resolves the FID on which the VNI should be
set and then sets the VNI. Currently, the FID is simply resolved
according to the ifindex of the bridge device to which the VxLAN device
is enslaved. This works because only VLAN-unaware bridges are supported.

With VLAN-aware bridges the FID would need to be resolved based on the
VLAN to which the VNI is mapped to.

Add the VLAN ID to the argument list of 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>