drivers/video/via/via-gpio.c: fix warning

drivers/video/via/via-gpio.c: In function 'viafb_gpio_probe':
drivers/video/via/via-gpio.c:216: warning: assignment from incompatible pointer type

Cc: Joseph Chan <JosephChan@via.com.tw>
Cc: Scott Fang <ScottFang@viatech.com.cn>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>