net: sparx5: mdb add/del handle non-sparx5 devices

When adding/deleting mdb entries on other net_devices, eg., tap
interfaces, it should not crash.

Fixes: 3bacfccd

 ("net: sparx5: Add mdb handlers")
Signed-off-by: Casper Andersson <casper.casan@gmail.com>
Reviewed-by: Steen Hegelund <Steen.Hegelund@microchip.com>
Link: https://lore.kernel.org/r/20220630122226.316812-1-casper.casan@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>