gpio: sysfs: only get the dirent reference for the value attr once

gpio: sysfs: only get the dirent reference for the value attr once

There's no reason to retrieve the reference to the sysfs dirent every
time we request an interrupt, we can as well only do it once when
exporting the GPIO.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20250704-gpio-sysfs-chip-export-v4-3-9289d8758243@linaro.org
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>