usb: pd: Fix unintentional discarding of outgoing VDM

Currently when a VDM message arrives, any pending VDM TX message
queued will be aborted. But this will inadvertently discard an
initiated Discover Identity if an incoming message such as
Attention arrives just before it. Since there is no TX response
to Attention, the engine should have just been able to resume
the Discover ID message. In general, only discard VDM TX messages
when attempting to queue a new outgoing VDM.

Change-Id: Ife42a19b8f909b0550b6bd44ed470e87a922d6e3
Signed-off-by: Jack Pham <jackp@codeaurora.org>