greybus: loopback: add response len to loopback protocol

This patch adds a len field to the loopback protocol.
This field is validated in gb_loopback_transfer() and stuffed in
gb_loopback_request_recv().

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Reviewed-by: Patrick Titiano <ptitiano@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>