mac802154: fix atomic_dec_and_test checks

We need to call wake_up() when hold_txs reaches zero. The semantic of
atomic_dec_and_test() is that it returns true when it's zero.

Fixes: f0feb349 ("net: mac802154: Introduce a tx queue flushing mechanism")
Signed-off-by: Alexander Aring <aahringo@redhat.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20220613043735.1039895-3-aahringo@redhat.com


Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>