mtd: nand: denali: squash denali_enable_dma() helper into caller

This helper just sets/clears a flag of DMA_ENABLE register (with
register read-back, I do not know why it is necessary).

Move the register write code to the caller, and remove the helper.
It works for me without the register read-back.

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