Should return NETDEV_TX_{OK,BUSY} instead of 0,-1 (this doesn't change
any current functionality).

Changes-licensed-under: 3-Clause-BSD
Reported-by: NJohannes Berg <johannes@sipsolutions.net>
Signed-off-by: NBob Copeland <me@bobcopeland.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>