[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>