tridentfb: fix pseudo_palette array overrun in setcolreg

The pseudo_palette has only 16 elements. Do not write if regno (the array
index) is more than 15.

Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>