gpio: Remove VLA from MAX3191X driver

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

)

This patch replaces several a VLA with an appropriate call to
kmalloc_array.

Signed-off-by: Laura Abbott <labbott@redhat.com>
Reviewed-and-tested-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>