greybus: loopback: remove endo reference

Replace reference to "endo0" and generate the raw-latency filename based
on the host-device bus id instead.

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