mtd: nand: sh_flctl: handle dma_submit() errors

Some build tools noticed that 'cookie' is being set but not used. Might
as well catch the errors here and handle them the same way we handle
other DMA prep steps.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>