sisfb: fix color component length for pseudocolor modes

sisfb incorrectly sets the length of the color fields to 6 bits
for PSEUDOCOLOR modes, even though 8 bits are always used per pixel.
Fix this by setting the length to 8.

Signed-off-by: Michal Januszewski <spock@gentoo.org>
Cc: Thomas Winischhofer <thomas@winischhofer.net>
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>