mtd: nand: cafe: remove use of NAND_OWN_BUFFERS

This driver is the last/only user of NAND_OWN_BUFFERS.  Boris suggested
to remove this flag.

Taking a closer look at this driver, it calls dma_alloc_coherent() for
the concatenated area for the DMA bounce buffer + struct nand_buffers,
but the latter does not need to be DMA-coherent; cafe_{write,read}_buf
simply do memcpy() between buffers when usedma==1.

Let's do dma_alloc_coherent() for the DMA bounce buffer in the front,
and leave the nand_buffers allocation to nand_scan_tail(), then rip off
NAND_OWN_BUFFERS.

The magic number, 2112, is still mysterious (hard-coded writesize +
oobsize ?), but this is not our main interest.  I am keeping it.

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