greybus: gb_loopback: Fix throughput calculations

Throughput and requests per second calculations are broken for
asynchronous request.
Instead of calculate the throughput for each iteration,
calculate it once at the end of the test.

Signed-off-by: Alexandre Bailon <abailon@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>