base-call: listen for connection error URCs in-call

In order to detect state changes while we're in-call, we have to
monitor the standard connection errors we're used to, but have them
processed as URCs instead of as responses to ATD requests.

plugins/huawei/mm-call-huawei.c

@@ -139,5 +139,7 @@ mm_call_huawei_class_init (MMCallHuaweiClass *klass)
 {
     MMBaseCallClass *base_call_class = MM_BASE_CALL_CLASS (klass);
 
-    base_call_class->start = call_start;
+    base_call_class->start                      = call_start;
+    base_call_class->setup_unsolicited_events   = NULL;
+    base_call_class->cleanup_unsolicited_events = NULL;
 }