[media] omap3isp: preview: Skip brightness and contrast in configuration ioctl

Brightness and contrast are handled through V4L2 controls. Their
configuration bit in the preview engine update attributes table is set
to -1 to reflect that. However, the VIDIOC_OMAP3ISP_PRV_CFG ioctl
handler doesn't handle -1 correctly as a configuration bit value, and
erroneously considers that the parameter has been selected for update by
the ioctl caller. Fix this.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>