greybus: loopback: allocate a response even for a 0-byte request

If payload length of a transfer packet is 0, no response is allocated.
Send a well-formed response even in that case.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>