greybus: camera: Early update num_streams and flags

In configure_streams Operation, when returning from the greybus
operation call, we can assign the num_streams and flags variable earlier
so that if the following stream configuration inspection fails, the
caller receives the right number of configured streams anyway

Testing Done: White camera module preview and capture.
              Triggering failure during stream configuration inspection
              makes configuration fails, but avoid segfaulting as
              was previously happening

Signed-off-by: Jacopo Mondi <jacopo.mondi@linaro.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@linaro.org>
Signed-off-by: Alex Elder <elder@linaro.org>