ath5k: fix return values from ath5k_tx

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: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>