mtd: denali_dt: Change return value to fix smatch warning

platform_get_irq() also returns -ENXIO upon failure.
Use it instead of hardcoded return type.

Fixes the following smatch warning:
drivers/mtd/nand/denali_dt.c:93 denali_dt_probe() info:
why not propagate 'denali->irq' from platform_get_irq() instead of (-6)?

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>