iwlwifi: mvm: use debugfs_create_bool() for enable_scan_iteration_notif

There is no need to implement the enable_scan_iteration_notif handling
explicitly and there's no reason not to export the current value.  So
use debugfs_create_bool() instead.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>