cmd: change the return value when argc error

When the number of parameters is wrong, the return value should be processed in
the same way as other cmds, return CMD_RET_USAGE so that it can print the information.

Signed-off-by: Shenlin Liang <liangshenlin@eswincomputing.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/mvebu/comphy_rx_training.c b/cmd/mvebu/comphy_rx_training.c
index 25a9e15..4ee8f54 100644
--- a/cmd/mvebu/comphy_rx_training.c
+++ b/cmd/mvebu/comphy_rx_training.c
@@ -22,7 +22,7 @@
 
 	if (argc != 3) {
 		printf("missing arguments\n");
-		return -1;
+		return CMD_RET_USAGE;
 	}
 
 	cp_index = hextoul(argv[1], NULL);