mtd: nand: denali: rename misleading dma_buf to tmp_buf

The "dma_buf" is not used for a DMA bounce buffer, but for arranging
the transferred data for the syndrome page layout.  In fact, it is
used in the PIO mode as well, so "dma_buf" is a misleading name.
Rename it to "tmp_buf".

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