gpio: sysfs: don't look up exported lines as class devices

In preparation for adding a parallel, per-chip attribute group for
exported GPIO lines, stop using class device APIs to refer to it in the
code. When unregistering the chip, don't call class_find_device() but
instead store exported lines in a linked list inside the GPIO chip data
object and look it up there.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20250704-gpio-sysfs-chip-export-v4-7-9289d8758243@linaro.org


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