greybus: loopback: Fix calculations error for ping transfers

For the async ping transfer, statistics are counted twice,
once after the after the gb_loopback_async_operation() and
once in the callback.
Only keep the one in the callback.

Signed-off-by: Alexandre Bailon <abailon@baylibre.com>
Reported-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>