paging: Move the silent_call code to use the subscriber request

Coordinate with the normal subscriber channel requests instead
of going to page ourselves. This might lead to getting a channel
that is of a different type though.
diff --git a/openbsc/src/libmsc/silent_call.c b/openbsc/src/libmsc/silent_call.c
index 010c2b4..e9ece18 100644
--- a/openbsc/src/libmsc/silent_call.c
+++ b/openbsc/src/libmsc/silent_call.c
@@ -118,11 +118,10 @@
 /* initiate a silent call with a given subscriber */
 int gsm_silent_call_start(struct gsm_subscriber *subscr, void *data, int type)
 {
-	int rc;
+	struct subscr_request *req;
 
-	rc = paging_request(subscr->group->net, subscr, type,
-			    paging_cb_silent, data);
-	return rc;
+	req = subscr_request_channel(subscr, type, paging_cb_silent, data);
+	return req != NULL;
 }
 
 /* end a silent call with a given subscriber */