mtd: nand: sh_flctl: fix error return code in flctl_probe()

platform_get_irq() returns an error code, but the sh_flctl
driver ignores it and always returns -ENXIO. This is not correct
and, prevents -EPROBE_DEFER from being propagated properly.

Print and propagate the return value of platform_get_irq on failure.

This issue was detected with the help of Coccinelle.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>