mlxsw: spectrum_fid: Add APIs to lookup FID without creating it

Current APIs only allow looking for a FID and creating it in case it
does not exist.

With VxLAN, in case the bridge to which the VxLAN device was enslaved
does not already have a corresponding FID, then it means that something
went wrong that we need to be aware of.

Add an API to look up a FID, but without creating it in order to catch
above-mentioned situation.

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