mtd: nand: denali: use flag instead of register macro for direction

It is not a good idea to re-use macros that represent a specific
register bit field for the transfer direction.

It is true that bit 8 indicates the direction for the MAP10 pipeline
operation and the data DMA operation, but this is not valid across
the IP.

Use a simple flag (write: 1, read: 0) for the direction.

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