Revert "gpiolib: remove set but not used variable 'config'"

This reverts commit e5e42ad2.

This patch came on top of another patch that introduced a regression.
Revert it before addressing the culprit.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>