[VIDEO] sunxvr500fb: Fix pseudo_palette array size

- the pseudo_palette is only 16 elements long.
- 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: David S. Miller <davem@davemloft.net>