gpiolib: sysfs: Do unexport GPIO when user asks for it

It seems that sysfs interface implicitly relied on the gpiod_free() to
unexport the line. This is logically incorrect as core gpiolib should
not deal with sysfs so instead of restoring it, let's call
gpiod_unexport() from sysfs code.

Fixes: b0ce9ce4 ("gpiolib: Do not unexport GPIO on freeing")
Reported-by: Marek Behún <kabel@kernel.org>
Closes: https://lore.kernel.org/r/20230808102828.4a9eac09@dellmb


Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Marek Behún <kabel@kernel.org>
[Bartosz: tweaked the commit message]
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>