sisfb: eliminate compiler warnings

Convert some bit masks to 8-bit values to avoid the following compiler
warnings:

drivers/video/sis/sis_main.c:3858: warning: large integer implicitly truncated to unsigned type
drivers/video/sis/init301.c:5855: warning: large integer implicitly truncated to unsigned type
drivers/video/sis/init301.c:5869: warning: large integer implicitly truncated to unsigned type

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: Thomas Winischhofer <thomas@winischhofer.net>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>