iwlwifi: mvm: unref SCAN ref on scan completion

The scan reference wasn't released in case of offloaded scan,
causing the refcount to go wrong and prevent the device
from going into d0i3.

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>