iwlwifi: mvm: BT Coex - minor API change

The BT Coex API underwent a minor backward compatible API
change.
We now need to set an invalid value in the override fields.
While at it, add kerneldoc comments on the fields in the
command.

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