i2c: fix command usage help

Portions of this work were supported by funding from
the CE Linux Forum.

Signed-off-by: Heiko Schocher <hs@denx.de>

common/cmd_i2c.c

@@ -1288,6 +1288,9 @@ static int do_i2c(cmd_tbl_t * cmdtp, int flag, int argc, char * const argv[])
 {
 	cmd_tbl_t *c;
 
+	if (argc < 2)
+		return cmd_usage(cmdtp);
+
 	/* Strip off leading 'i2c' command argument */
 	argc--;
 	argv++;