iwlwifi: coex API data structure

Add data structure define for COEX_MEDIUM_NOTIFICATION and
COEX_EVENT_CMD host commands.

COEX_MEDIUM_NOTIFICATION is notification from uCode to host to indicate
medium changes. COEX_EVENT_CMD is from host to uCode for coex event
request.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>