net: ieee802154: fix null deref in parse dev addr

Fix a logic error that could result in a null deref if the user sets
the mode incorrectly for the given addr type.

Signed-off-by: Dan Robertson <dan@dlrobertson.com>
Acked-by: Alexander Aring <aahringo@redhat.com>
Link: https://lore.kernel.org/r/20210423040214.15438-2-dan@dlrobertson.com


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