mtd: nand: squash struct nand_buffers into struct nand_chip

struct nand_buffers is malloc'ed in nand_scan_tail() just for
containing three pointers.  Squash this struct into nand_chip.

Move and rename as follows:

  chip->buffers->ecccalc   ->  chip->ecc.calc_buf
  chip->buffers->ecccode   ->  chip->ecc.code_buf
  chip->buffers->databuf   ->  chip->data_buf

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