usb: dwc3: core: add ep0_next_event field

this field will hold the next expected event.

In certain cases, host might fall into some error
condition and ask from us the wrong Control phase.
On such situations, we should stall and restart.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Ido Shayevitz <idos@codeaurora.org>