osmo-bsc: fix compiler warning: store struct in vty->index

Don't store an MSC index number in the vty->index void* value. Instead,
store the osmo_msc_data struct directly. Thus avoid warnings about
differences in int vs void* sizes, and save some index lookups.
diff --git a/openbsc/src/osmo-bsc/osmo_bsc_vty.c b/openbsc/src/osmo-bsc/osmo_bsc_vty.c
index d871f01..e623c9c 100644
--- a/openbsc/src/osmo-bsc/osmo_bsc_vty.c
+++ b/openbsc/src/osmo-bsc/osmo_bsc_vty.c
@@ -43,7 +43,7 @@
 
 static struct osmo_msc_data *osmo_msc_data(struct vty *vty)
 {
-	return osmo_msc_data_find(bsc_gsmnet, (int) vty->index);
+	return vty->index;
 }
 
 static struct cmd_node bsc_node = {
@@ -70,7 +70,7 @@
 		return CMD_WARNING;
 	}
 
-	vty->index = (void *) index;
+	vty->index = msc;
 	vty->node = MSC_NODE;
 	return CMD_SUCCESS;
 }