video: 68328fb: remove check for CONFIG_FB_68328_INVERT

Since v2.6.7 the 68328 frame buffer driver contains a check for
CONFIG_FB_68328_INVERT. But the Kconfig symbol FB_68328_INVERT was never
added to the tree. Remove this check and do some related cleaning up.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>