[MTD] [OneNAND] Use the u_char instead of char in oobbuf

In function onenand_verify_oob, local variable oobbuf shall be unsigned char.

In the case of a value is >= 0x80, it's unequal in comparing the value in an unsigned char and signed char.
Signed-off-by: Sheng Yongjie (Sam) <samsheng@trident.com.cn>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>

drivers/mtd/onenand/onenand_base.c

@@ -1217,7 +1217,7 @@ int onenand_bbt_read_oob(struct mtd_info *mtd, loff_t from,
 static int onenand_verify_oob(struct mtd_info *mtd, const u_char *buf, loff_t to)
 {
 	struct onenand_chip *this = mtd->priv;
-	char oobbuf[64];
+	u_char oobbuf[64];
 	int status, i;
 
 	this->command(mtd, ONENAND_CMD_READOOB, to, mtd->oobsize);