greybus: loopback: Add reserved fields to transfer request

All loopback transfer operations should have an identical header
format in order to facilitate bandwidth and data movement analysis.

Suggested-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Axel Haslam <ahaslam@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>