mtd: rawnand: denali: fix a race condition when DMA is kicked

I thought the read-back of the DMA_ENABLE register was unnecessary
(at least it is working on my boards), then deleted it in commit
586a2c52 ("mtd: nand: denali: squash denali_enable_dma() helper
into caller").  Sorry, I was wrong - it caused a timing issue on
Cyclone5 SoCFPGAs.

Revive the register read-back, commenting why this is necessary.

Fixes: 586a2c52 ("mtd: nand: denali: squash denali_enable_dma() helper into caller")
Cc: <stable@vger.kernel.org>
Reported-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>