ath5k: properly drop packets from ops->tx

We shouldn't return NETDEV_TX_BUSY from the TX callback, especially
after we've mucked with the sk_buffs.  Drop the packets and return
NETDEV_TX_OK.

Changes-licensed-under: 3-Clause-BSD

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>