gpio: sysfs: don't use driver data in sysfs callbacks for line attributes

gpio: sysfs: don't use driver data in sysfs callbacks for line attributes

Currently each exported GPIO is represented in sysfs as a separate class
device. This allows us to simply use dev_get_drvdata() to retrieve the
pointer passed to device_create_with_groups() from sysfs ops callbacks.

However, we're preparing to add a parallel set of per-line sysfs
attributes that will live inside the associated gpiochip group. They are
not registered as class devices and so have the parent device passed as
argument to their callbacks (the GPIO chip class device).

Put the attribute structs inside the GPIO descriptor data and
dereference the relevant ones using container_of() in the callbacks.
This way, we'll be able to reuse the same code for both the legacy and
new GPIO attributes.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20250704-gpio-sysfs-chip-export-v4-6-9289d8758243@linaro.org
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>