gpio: sifive: Directly use the device's fwnode

There is no need to convert dev->of_node back to a fwnode_handle.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>