greybus: 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.
In addition, update every seconds the min and the max
for throughput and requests per second.

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