greybus: camera: Return the result flags from the configure_streams response

And return the response num_streams field through a pointer variable
instead of using the return value of the function as both an error code
and a positive number of streams, it makes the API more consistent.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>