ath9k: correct warning about unintialized variable 'tid'

drivers/net/wireless/ath9k/xmit.c: In function ‘ath_tx_start’:
drivers/net/wireless/ath9k/xmit.c:1858: warning: ‘tid’ may be used uninitialized in this function

Signed-off-by: John W. Linville <linville@tuxdriver.com>