gpio: sysfs: fix inverted pointer logic

The logic is inverted, we want to return if the chip *IS* NULL.

Fixes: d83cee3d

 ("gpio: protect the pointer to gpio_chip in gpio_device with SRCU")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/linux-gpio/15671341-0b29-40e0-b487-0a4cdc414d8e@moroto.mountain/


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