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: Luis R. Rodriguez <mcgrof@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>