greybus: es2: Send cport-id in wValue field to usb_control_msg()
wIndex field has a special meaning, as that can be used by the core to
index into the possible USB interfaces. And that specifically broke with
gbsim, as it has a single USB interface.
Other similar requests (REQUEST_LATENCY_TAG_{EN|DIS}) are already using
wValue field for passing cport-id.
Fix cport_reset() by sending the cport-id in wValue field instead of
wIndex.