gpiolib: remove set but not used variable 'config'

gpiolib: remove set but not used variable 'config'

drivers/gpio/gpiolib.c: In function gpio_set_config:
drivers/gpio/gpiolib.c:3053:16: warning:
 variable config set but not used [-Wunused-but-set-variable]

commit d90f36851d65 ("gpiolib: have a single place
of calling set_config()") left behind this unused variable.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Link: https://lore.kernel.org/r/20200108121117.45060-1-yuehaibing@huawei.com
Reviewed-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>