gpio: use sizeof() instead of hardcoded values

gpiolib uses a fixed string for the suffixes and defines it at 32 bytes.
Later in the code snprintf is used with this fixed value of 32. Using
sizeof() is safer in case the size for the suffixes is ever changed.

Signed-off-by: Olliver Schinagl <oliver@schinagl.nl>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>