staging/xgifb: Remove always false comparisons

This patch removes some comparisons that always evaluate to false since
xoffset and yoffset are defined as __u32 in fb_var_screeninfo in
include/linux/fb.h and thus can never be negative.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>