gluon-setup-mode: set regdom

OpenWrt doesn't set the regdom unless a WLAN device is started. Explicitly set
the regdom in the setup mode to get the correct list of allowed txpower entries
in the planned LuCI module.

package/gluon-setup-mode/files/lib/gluon/setup-mode/rc.d/S20network

@@ -45,6 +45,7 @@ init_switch() {
 start_service() {
 	prepare_config
 	init_switch
+	iw reg set "$(lua -e 'print(require("gluon.site_config").regdom)')"
 
 	procd_open_instance
 	procd_set_param command /sbin/netifd -c /var/gluon/setup-mode/config