mlxsw: spectrum_flower: Do not force matching on iif

Currently, mlxsw only supports the 'ingress_ifindex' field in the
'FLOW_DISSECTOR_KEY_META' key, but subsequent patches are going to add
support for the 'l2_miss' field as well. It is valid to only match on
'l2_miss' without 'ingress_ifindex', so do not force matching on it.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>