ath5k: Use QUIET mechanism on tx dma stop

 * Use QUIET mechanism to drain tx buffer on PCU for newer chips
 * Make sure that INTPEND is really 1 and not 0xffffffff while checking for pending interrupts

Changes-Licensed-under: ISC
Signed-Off-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>