sisfb: eliminate compiler warnings

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 <[email protected]>
Cc: Thomas Winischhofer <[email protected]>
Signed-off-by: Paul Mundt <[email protected]>