viafb: replace strict_strtoul to kstrto* and check return value

This commit replace the function strict_strtoul(becasue commit 33ee3b2e), and check the return value to avoid such warning:

  drivers/video/via/viafbdev.c:1992: warning: ignoring return value of 'kstrtoul', declared with attribute warn_unused_result

Signed-off-by: Wang Shaoyan <wangshaoyan.pt@taobao.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>