huawei: use AT^SYSCFG for radio setting operations on 3G only modems

AT^SYSCFGEX must be used on LTE Huawei modems to enable LTE support.
But some modems (or firmwares?) do not support this command and AT^SYSCFG
must be used to get/set radio settings.
This has been introduced in commit:
22adf6402c

There is a bug in this commit and AT^SYSCFGEX commands are used even on
modems not supporting it.

drivers/huaweimodem/radio-settings.c

@@ -532,6 +532,7 @@ static void syscfgex_support_cb(gboolean ok, GAtResult *result,
 	if (!ok) {
 		g_at_chat_send(rsd->chat, "AT^SYSCFG=?", syscfg_prefix,
 					syscfg_support_cb, rs, NULL);
+		return;
 	}
 
 	rsd->syscfgex_cap = 1;