[paging] Start with a smaller paging limit...

The value 20 is just a random number and it really depends
on the number of TRX on a bts to be a sane or insane limit.
diff --git a/openbsc/src/paging.c b/openbsc/src/paging.c
index e0cb531..9a4f365 100644
--- a/openbsc/src/paging.c
+++ b/openbsc/src/paging.c
@@ -167,7 +167,7 @@
 	bts->paging.work_timer.data = &bts->paging;
 
 	/* Large number, until we get a proper message */
-	bts->paging.available_slots = 100;
+	bts->paging.available_slots = 20;
 }
 
 static int paging_pending_request(struct gsm_bts_paging_state *bts,