usb: dwc3: ep0: prevent starting transfers twice on ep0

In case we try to start an invalid test mode, we
will call dwc3_ep0_stall_and_restart() but we will
also call dwc3_ep0_out_start() which will start
a second transfer on ep0.

Let's prevent any problems by returning early in
the error case.

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