mlxsw: spectrum_fid: Allow FID lookup by its index

When processing a notification about a new FDB entry learned from a
VxLAN tunnel, the driver is provided with the FID index among other
parameters.

The driver potentially needs to update the bridge and VxLAN drivers
about the new entry using a pointer to the VxLAN device and the
corresponding VNI.

These two parameters are stored in the FID, so add a new function that
allows looking up a FID based on its index.

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