usb: dwc3: ep0: Fail enqueue() when racing with vbus_sess disconnect

When the setup packet comes right before the disconnect notification
there is a possible race between clearing the run stop bit and
enqueuing the setup packet. This results in controller halt failure.
Add a vbus_active check before enqueuing the packet to fix this issue.

Change-Id: I3921a1fa92a0f5c6892f1cc3ffe9dda6366b72b1
Signed-off-by: Elson Roy Serrao <eserrao@codeaurora.org>