mlxsw: spectrum_flower: Set dummy FID before forward action

HW requires the FID to be valid in order for the forward action to work.
So regardless of the current FID validity, just set the dummy FID which
would do the trick.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>