net: mac802154: Save a global error code on transmissions

So far no error is returned from a failing transmission. However it
might sometimes be useful, and particularly easy to use during sync
transfers (for certain MLME commands). Let's create an internal variable
for that, global to the device. Right now only success are registered,
which is rather useless, but soon we will have more situations filling
this field.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Alexander Aring <aahringo@redhat.com>
Link: https://lore.kernel.org/r/20220407100903.1695973-4-miquel.raynal@bootlin.com


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