gpio: sysfs: Obey valid_mask

Do not allow exporting GPIOs which are set invalid
by the driver's valid mask.

Fixes: 726cb3ba ("gpiolib: Support 'gpio-reserved-ranges' property")
Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>