nfp: flower: make nfp_fl_lag_changels_event() void

nfp_fl_lag_changels_event() never fails, and therefore we would
never return NOTIFY_BAD for NETDEV_CHANGELOWERSTATE.  Make this
clearer by changing nfp_fl_lag_changels_event()'s return type
to void.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: John Hurley <john.hurley@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>