viafb: make it work on x86_64

Fix a bug in viafb on x86_64 builds (e.g. for VIA Nano CPU).

You cannot make the assumption that sizeof(unsigned int) ==
sizeof(unsigned long), so the parsing of the default mode (640x480) fails,
leading to a division by zero during insmod of the driver.

Signed-off-by: Harald Welte <HaraldWelte@viatech.com>
Cc: <JosephChan@via.com.tw>
Cc: <ScottFang@viatech.com.cn>
Cc: Acked-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>