pdch: Move paging dequeue into the PDCH object

Rely on packet_paging_request returning NULL in case the queue
is empty. We should move the write_packet_paging_request into
a separate file/object as well.
diff --git a/src/gprs_rlcmac_sched.cpp b/src/gprs_rlcmac_sched.cpp
index c0154fc..be699e6 100644
--- a/src/gprs_rlcmac_sched.cpp
+++ b/src/gprs_rlcmac_sched.cpp
@@ -156,8 +156,7 @@
 		return msg;
 	}
 	/* schedule PACKET PAGING REQUEST */
-	if (!llist_empty(&pdch->paging_list))
-		msg = gprs_rlcmac_send_packet_paging_request(pdch);
+	msg = pdch->packet_paging_request();
 	if (msg) {
 		LOGP(DRLCMACSCHED, LOGL_DEBUG, "Scheduling paging request "
 			"message at RTS for (TRX=%d, TS=%d)\n", trx, ts);