usb: pd: Respond NAK to Discover Identity

There is no known usage or requirement for our response to
Discover Identity VDM when a UFP, which currently uses hard-
coded values, nor is the XID a field we have use for. Instead
simply respond with a NAK. This requires the VIF to be updated
with 'Responds_To_Discov_SOP_UFP=NO'.

Change-Id: I3033054e2b7b511515d5f11ba17ff68f11465902
Signed-off-by: Jack Pham <jackp@codeaurora.org>