ath9k_htc: Simplify TX URB management

This patch simplifies URB management for transmission,
by removing the 'FLUSH' variable (which is not needed,
since we can determine if the URB has been killed by
looking at the URB status), and also handling the STOP
case properly.

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