In a similar fashion to commit 564c6d72 ("mlxsw: spectrum_fid: Add
APIs to lookup FID without creating it"), add a corresponding API to
lookup 802.1Q FIDs.

This is a prerequisite to VxLAN support with VLAN-aware bridges and will
allow us to resolve a 802.1Q FID by its VLAN when an FDB entry is added
on the bridge port of the VxLAN device.
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>