macfb: Do not overflow fb_fix_screeninfo.id

Don't overflow the 16-character fb_fix_screeninfo id string (fixes some
console erasing and blanking artifacts). Have the ID default to "Unknown"
on machines with no built-in video and no nubus devices. Check for
fb_alloc_cmap failure.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>