greybus: loopback: send command to APBridge to tag throughput

A USB vendor command has been added to APBridge to allow for tagging of
specific CPort identifiers with internal timing data, specifically geared
towards capturing and understanding latencies in the UniPro fabric. This
patch sends a command to APBridge for each known loopback CPort to tag data
as appropriate. Subsequent patches will present this data to user-space for
ongoing integration analysis.

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