ath: document ATH_DBG_MCI

A debug level was added to the ath module for printing
MCI messages but no documentation was provided. Clarify that
MCI is the Message Coexistence Interface, a private protocol
used exclusively for WLAN-BT coexistence starting from
AR9462.

Cc: wtsao@qca.qualcomm.com
Cc: rmanohar@qca.qualcomm.com
Cc: mohammed@qca.qualcomm.com
Cc: senthilb@qca.qualcomm.com
Signed-off-by: NLuis R. Rodriguez <mcgrof@qca.qualcomm.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath/ath.h

@@ -215,6 +215,9 @@ do {								\
  * @ATH_DBG_HWTIMER: hardware timer handling
  * @ATH_DBG_BTCOEX: bluetooth coexistance
  * @ATH_DBG_BSTUCK: stuck beacons
+ * @ATH_DBG_MCI: Message Coexistence Interface, a private protocol
+ *	used exclusively for WLAN-BT coexistence starting from
+ *	AR9462.
  * @ATH_DBG_ANY: enable all debugging
  *
  * The debug level is used to control the amount and type of debugging output