usb: dwc3: ep0: rename dwc3_ep0_complete_req to dwc3_ep0_complete_status

That's a much more intuitive name as that function
is only called at the completion of a Status Phase.

It also matches dwc3_ep0_complete_data() for
the completion of Data Phase.

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