gpio: Remove VLA from stmpe driver

The new challenge is to remove VLAs from the kernel
(see https://lkml.org/lkml/2018/3/7/621

)

The number of GPIOs on the supported chips is fairly small
so stack allocate to a known upper bound and spit out a warning
if any new chips have more gpios.

Signed-off-by: Laura Abbott <labbott@redhat.com>
Reviewed-by: Phil Reid <preid@electromag.com.au>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>