macfb: fix 24-bit visual and stuff

Lots of changes. Remove volatile keywords. Take the advice in the comments
and change video_slot to slot_addr. Factor out fifteen or so "channel >>
8" shifts into three shifts higher up the call graph. Make
csc_setpalette() atomic, pass the correct color values. Check for
ioremap() failure. Add missing break statement, thus fixing 24 bit console
visual (blue background bug). Remove some uninformative printk() noise.
Add a description for powerbook 140/170 graphics.
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>