mtd: nand: denali: merge struct nand_buf into struct denali_nand_info

Now struct nand_buf has only two members, so I see no reason for the
separation.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>