iwlwifi: mvm: BT Coex - enable Tx power based on BT status

The activity grading indication from the firmware should
not be used in this case, but the bt_status in the firwmare
notification.
Fix that.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>