nfp: flower: don't try to nack device unregister events

Returning an error from a notifier means we want to veto the change.
We shouldn't veto NETDEV_UNREGISTER just because we couldn't find
the tracking info for given master.

I can't seem to find a way to trigger this unless we have some
other bug, so it's probably not fix-worthy.

While at it move the checking if the netdev really is of interest
into the handling functions, like we do for other events.

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>