gsm_04_11: Start using the GSM Subscriber class to ask for a channel

The paging message should not be called directly and the GSM Subscriber
can handle multiple requests at the same time... Now a subscr_put_channel
should be called after the message sending. But it is not very clear when
this can be called. The current code works by luck that the SAPI=0 will
be released...
The MT-SMS was tested via the VTY interface and a N900.
diff --git a/openbsc/src/gsm_04_11.c b/openbsc/src/gsm_04_11.c
index 3aa9d09..a346913 100644
--- a/openbsc/src/gsm_04_11.c
+++ b/openbsc/src/gsm_04_11.c
@@ -1186,11 +1186,7 @@
 				     rll_ind_cb, sms);
 
 	/* if not, we have to start paging */
-	rc = paging_request(subscr->net, subscr, RSL_CHANNEED_SDCCH,
-			    paging_cb_send_sms, sms);
-	if (rc <= 0)
-		sms_free(sms);
-
+	subscr_get_channel(subscr, RSL_CHANNEED_SDCCH, paging_cb_send_sms, sms);
 	return 0;
 }