video: pxa3xx-gcu: provide an empty .open call

This is necessary in order to make the core set file->private_data to
miscdev in use. We need that information later to dereference the
container of the device, so we can get access to our private struct from
other callbacks.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Acked-by: Haojian Zhuang <haojian.zhuang@gmail.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>