mtd: nand: denali: slight clean up of denali_wait_for_irq()

This function has a local variable "irq_mask" and its value is
the same as denali->irq_mask.  Clean up the code a little.

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