fbdev: fbmem: Fix logo center image dx issue

Image.dx gets wrong value because of missing '()'.

If xres == logo->width and n == 1, image.dx = -16.

Signed-off-by: Guiling Deng <greens9@163.com>
Fixes: 3d8b1933

 ("fbdev: fbmem: add config option to center the bootup logo")
Cc: stable@vger.kernel.org # v5.0+
Signed-off-by: Helge Deller <deller@gmx.de>