video: fbdev: atari: Fix inverse handling

Currently, the "inverse" option does not do anything, as it just sets a
flag, which is further unused.

Fix this by calling fb_invert_cmaps() instead, like other drivers do.
As this only affects the console colormap, this does not affect X.
Update the documentation to match the actual behavior.

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