mlxsw: spectrum_switchdev: Avoid leaking FID's reference count

It should never be possible for a user to set a VNI on a FID in case one
is already set. The driver therefore returns an error, but fails to drop
the reference count taken earlier when calling
mlxsw_sp_fid_8021d_lookup().

Drop the reference when this unlikely error is hit.

Fixes: 1c30d183

 ("mlxsw: spectrum: Enable VxLAN enslavement to bridges")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>