viafb: fix pointer type missmatch

  drivers/video/via/via-core.c: In function 'via_pci_probe':
  drivers/video/via/via-core.c:691: warning: assignment discards qualifiers from pointer target type

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