usb: pd: Do not respond to SVDM messages in PD 2.0

Since our device does not support responding to Structured VDMs
(VIF Responds_to_Discov_SOP_DFP and Responds_to_Discov_SOP_UFP
fields are both false), any initiated discovery messages must
simply be ignored rather than responding with a NAK.

This addresses test failures observed in TEST.PD.VDM.SRC.1,
TEST.PD.VDM.SRC.2, TEST.PD.VDM.SNK.1, TEST.PD.VDM.SNK.2,
and TEST.PD.VDM.SNK.5 of the USB PD Merged tests.

Change-Id: I70f97563e528e4cf6d2a3bce31af7a7c702d4824
Signed-off-by: Jack Pham <jackp@codeaurora.org>