cmd: i2c: fix coverity issue

coverity scan reported issue:
** CID 583415:       Integer handling issues  (INTEGER_OVERFLOW)
/cmd/i2c.c: 369           in do_i2c_write()

change the length variable into type int.

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

cmd/i2c.c

@@ -299,7 +299,8 @@ static int do_i2c_write(struct cmd_tbl *cmdtp, int flag, int argc,
 			char *const argv[])
 {
 	uint	chip;
-	uint	devaddr, length;
+	uint	devaddr;
+	int length;
 	int alen;
 	u_char  *memaddr;
 	int ret;