sms: Make the max_pending tunable via a vty runtime setting

Make it possible to change the max pending via the vty. This
can be useful to play with the performance of the queue.
diff --git a/openbsc/src/sms_queue.c b/openbsc/src/sms_queue.c
index 1161b6c..4ffde62 100644
--- a/openbsc/src/sms_queue.c
+++ b/openbsc/src/sms_queue.c
@@ -407,3 +407,11 @@
 			pending->subscr->id, VTY_NEWLINE);
 	return 0;
 }
+
+int sms_queue_set_max_pending(struct gsm_sms_queue *smsq, int max_pending)
+{
+	LOGP(DSMS, LOGL_NOTICE, "SMSqueue old max: %d new: %d\n",
+	     smsq->max_pending, max_pending);
+	smsq->max_pending = max_pending;
+	return 0;
+}