ath9k: Restart xmit logic in xmit watchdog.

The system can get into a state where the xmit queue
is stopped, but there are no packets pending, so
the queue will not be restarted.

Add logic to the xmit watchdog to attempt to restart
the xmit logic if this situation is detected.

Example 'dmesg' output:

ath: txq: f4e723e0 axq_qnum: 2, mac80211_qnum: 2 axq_link: f4e996c8 pending frames: 1 axq_acq empty: 1 stopped: 0 axq_depth: 0  Attempting to restart tx logic.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>