iwlwifi: mvm: add D0i3 ref/unref for scan

Take a reference when starting to scan and release it on completion.
Note that if the scan is cancelled/aborted, a completion will still be
sent up.

Signed-off-by: Arik Nemtsov <arikx.nemtsov@intel.com>
Signed-off-by: Eliad Peller <eliadx.peller@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>