ath9k_htc_tx_get_slot can return zero as valid index.
Signed-off-by: Rajkumar Manoharan <rmanoharan@atheros.com>
Acked-by: Sujith Manoharan <Sujith.Manoharan@Atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>