ath9k: Fix TX hang issue with Atheros chipsets

The hardware doesn't generate interrupts in some cases and so work
around this by monitoring the TX status periodically and reset the
chip if required.

This behavior of the hardware not generating the TX interrupts can
be noticed through ath9k debugfs interrupt statistics when heavy
traffic is being sent from STA to AP. One can easily see this behavior
when the STA is transmitting at a higher rates. The interrupt statistics
in the debugfs interface clearly shows that only RX interrupts alone
being generated and TX being stuck.

TX should be monitored through a timer and reset the chip only when
frames are queued to the hardware but TX interrupts are not generated
for the same even after one second. Also, we shouldn't remove holding
descriptor from AC queue if it happens to be the only descriptor and
schedule TX aggregation regarless of queue depth as it improves
scheduling of AMPDUs from software to hardware queue.

Signed-off-by: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: Senthil Balasubramanian <senthilkumar@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>