tridentfb: fix 224 color logo at 8 bpp

Fix depth setting for 8 bpp mode.  The nice 224 color logo is not
displayed in 8 bpp depth without this fix.

Signed-off-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>