net: mac802154: Fix a Tx warning check

The purpose of the netif_is_down() helper was to ensure that the network
interface used was still up when performing the transmission. What it
actually did was to check if _all_ interfaces were up. This was not
noticed at that time because I did not use interfaces at all before
discussing with Alexander Aring about how to handle coordinators
properly.

Drop the helper and call netif_running() on the right sub interface
object directly.

Fixes: 4f790184 ("net: mac802154: Add a warning in the slow path")
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Alexander Aring <aahringo@redhat.com>
Link: https://lore.kernel.org/r/20220617192914.1275611-1-miquel.raynal@bootlin.com


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