iwlwifi: mvm: Fix building channels in scan_config_cmd

Use the chanel hw_value and not the center frequency when
building channel array for scan_config_cmd.

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