ath9k: Merge queue draining functions

The TX queue draining routines have confusing names,
rename them approprately and merge ath_drain_txdataq()
with ath_drain_all_txq().

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