gpio: sysfs: rename the data variable in gpiod_(un)export()

In preparation for future commits which will make use of descriptor AND
GPIO-device data in the same functions rename the former from data to
desc_data separately which will make future changes smaller and easier
to read.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20250704-gpio-sysfs-chip-export-v4-5-9289d8758243@linaro.org


Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>