usb: dwc3: ep0: simplify EP0 state machine

The DesignWare USB3 core tells us which phase
of a control transfer should be started, it
also tells us which physical endpoint needs
that transfer.

With these two informations, we have all we
need to simply EP0 handling quite a lot and
get rid rid of the SW state machine tracking
ep0 states.

For achieving this perfectly, we needed to
add support for situations where we get
XferNotReady while endpoint is still busy
and XferNotReady while gadget driver still
hasn't queued a request.

Signed-off-by: Felipe Balbi <balbi@ti.com>