ath9k: Process GTT interrupts

Global Transmission Timeout interrupts are generated by
the HW when transmission of a frame fails - this is done
based on the threshold programmed in the AR_GTXTO register.

Currently, even though the interrupt is enabled for all chips,
it is not handled in the driver. This patch handles GTT events
for AR9003 and above chips, checking if the MAC/BB has hung
after successive GTT interrupts crosses a threshold (5).

This can be enabled for the older chips in the AR9002 family once
appropriate HW hang checks are implemented for them.

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