greybus: es1/es2: set transfer flag to send a zero-length packet

Greybus messages with a multiple size of 512B generate timeouts
(any other message size doesn't).
512B is exactly the packet size of a bulk out endpoint.
Hence USB device is expecting a short (< 512B)
or zero-length packet to finish the transfer,
which is never generated and causes the timeout.

Set the transfer flag to send a zero-length packet in this situation.

Signed-off-by: Alexandre Bailon <abailon@baylibre.com>
Reviewed-by: Patrick Titiano <ptitiano@baylibre.com>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>