usb: dwc3: ep0: introduce ep0_expect_in flag

This flag will tell us which direction we're
expecting on the next (data or status) phase.

It will help us catching errors of host going
crazy and requesting data of the wrong direction.

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