greybus: loopback: Fix incoherency in calculations in the case of error

Currently, in case the case of error, statistics are updated for
asynchronous but not for an asynchronous operation.
Do not update the statistics in the case of error.

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