usb: dwc3: ep0: Clear started flag on completion

Clear ep0's DWC3_EP_TRANSFER_STARTED flag if the END_TRANSFER command is
completed. Otherwise, we can't start control transfer again after
END_TRANSFER.

Cc: stable@vger.kernel.org
Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>