staging: xgifb: eliminate redudant chip type >= XG40 checks

Since all chips supported by the driver are >= XG40, these checks are
redundant and the code can be modified accordingly.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>