gpio: sifive: add missing check for platform_get_irq

Add the missing check for platform_get_irq() and return error code
if it fails.

The returned error code will be dealed with in
builtin_platform_driver(sifive_gpio_driver) and the driver will not
be registered.

Fixes: f52d6d8b

 ("gpio: sifive: To get gpio irq offset from device tree data")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>