greybus: loopback: fix invalid response size

The size of timestamps is not taken into account, which makes the
loopback driver in the firmware drop invalid packages.

Signed-off-by: Alexandre Bailon <abailon@baylibre.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>