iwlwifi: mvm: add UMAC scan iteration complete notification

Add UMAC scan iteration complete notification. This notification can
be enabled by setting scan_iter_notif_enabled through debugfs.
Upon receiving this notification, print the list of channels that
have been scanned in this iteration. This is useful for debugging.

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