iwlwifi: include rssi as part of decision making for reduce txpower

In bt coex, consider the average rssi as part of decision making process

Change-Id: I8d11d7f177a6875e2a9d08f7539d42253226fd7a
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Reviewed-on: http://git-mwg.jer.intel.com/gerrit/1945


Tested-by: Jenkins
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>