[media] omap3isp: preview: Remove update_attrs feature_bit field

The feature_bit is always equal to 1 << array position, remove it and
compute the value dynamically.

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>