gpio/via: rename VIA local config struct

The local VIA GPIO config struct "gpio_config" collides with my
extension to the gpio_chip struct, so rename it to
viafb_gpio_config so we don't get this clash.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>