gpiolib: Don't WARN on gpiod_put() for optional GPIO

In case of debug and optional GPIO requested, the gpiod_put() is not aware of
and will WARN, which is not the case.

Make gpiod_put() NULL-aware to keep silent for optional GPIOs.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>