usb: pd: Simplify VDM tx handling

VDMs, especially structured ones, are almost always handled
as requests and responses. Hence it does not make sense to
allow more than one outgoing VDM at a time, so get rid of the
vdm_tx_queue list and simplify it to a single-issued packet.

Because port partners can only have one VDM request/response in
flight, also handle the case when a new structured request is
received before the previous response has been sent. In that
case we simply discard the queued message as it's now invalid.

Change-Id: I144f8158dbf8be0babf5516b01d084fd053413c3
Signed-off-by: Jack Pham <jackp@codeaurora.org>