vty: Make sure to save timeslot mode in lowercase (as vty expects)

Change-Id: I9b11fbe3bf85b7096a3bd5f9b67168350e66a90a
diff --git a/src/vty.c b/src/vty.c
index 9dc548b..689aec7 100644
--- a/src/vty.c
+++ b/src/vty.c
@@ -176,7 +176,7 @@
 		mode_str = get_value_string(e1inp_ts_type_names, ts->type);
 
 		vty_out(vty, " line %u ts %u mode %s%s",
-			lnr, ts->num, mode_str, VTY_NEWLINE);
+			lnr, ts->num, osmo_str_tolower(mode_str), VTY_NEWLINE);
 	}
 }