ath9k: Fix the 'xmit' debugfs file

The 'xmit' debugfs file has become big and unwieldy, fix
multiple issues with its usage:

* Store TX counters/statistics only for the 4 Access Categories.
  Use IEEE80211_NUM_ACS instead of ATH9K_NUM_TX_QUEUES.

* Move various utility macros to debug.h, they can be reused
  elsewhere.

* Remove tx_complete_poll_work_seen.

* Remove code that accesses various internal queue-specific
  variables without any locking whatsoever. HW/SW queue details
  will be handled in a subsequent patch.

* Do not print internal values like txq_headidx and txq_headidx.
  They were mostly unused anyway, considering code like:
  PRX("txq_tailidx:     ", txq_headidx);

* Handle 'txprocdesc' for EDMA too.

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