usb: pd: Stop processing SVDM if handler found

If a client matching the SVID for an incoming SVDM is found,
assume it has handled the message completely and simply return.
This avoids the case when it handles an Attention message by
queuing another VDM request, only to have it inadvertently
deleted in the generic handling below.

But we also lose tracking of Discover/Enter/Exit Mode
exchanges though that was only used for informational purposes.

Change-Id: Ie23d473302d7fbda9b243a150e5c52d025007e4f
Signed-off-by: Jack Pham <jackp@codeaurora.org>