greybus: loopback: update throughput metrics to improve granularity

Throughput capture should account for the entire size of the data going out
on the wire. In addition throughput should be captured for each supported
loopback operation.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>