gpio:gpiolib: use static const char const * for a suffixes array

Checkpatch complains, and probably with good reason that we should use
const char const * for the static constant array that never gets
changed.

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