gpio-ml-ioh: fix error return code in ioh_gpio_probe()

Fix to return a negative error code in the irq descs alloc error
handling case instead of 0, as done elsewhere in this function.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>