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>

cmd/mvebu/comphy_rx_training.c

@@ -22,7 +22,7 @@ int mvebu_comphy_rx_training_cmd(struct cmd_tbl *cmdtp, int flag, int argc,
 
 	if (argc != 3) {
 		printf("missing arguments\n");
-		return -1;
+		return CMD_RET_USAGE;
 	}
 
 	cp_index = hextoul(argv[1], NULL);