fbdev: atafb - add palette register check

Add check if palette register number is in correct range
for few drivers which miss it. The regno value comes
indirectly from user space.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>