video: fbdev: atari: Fix ext_setcolreg()

The red, green, and blue color values are 16-bit, while the external
graphics hardware registers are 8-bit.
Add the missing conversion from 16-bit to 8-bit.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Tested-by: Michael Schmitz <schmitzmic@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>