gpio: mockup: Drop duplicate NULL check in gpio_mockup_unregister_pdevs()

Since platform_device_unregister() is NULL-aware, we don't need to duplicate
this check. Remove it and fold the rest of the code.

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