ath5k: Reset Tx interrupt bits also on PISR

Some cards don't update the PISR properly when all SISR bits
for Tx interrupts are being cleared and as a result we get
interrupt storm. Since we handle all tx queues all together
(so we don't really use the SISR bits to do per-queue interrupt
handling), we can manualy update PISR by doing a write-to-clear
on its Tx interrupt bits.

Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>