usb: dwc3: gadget: Clear started flag for non-IOC

Normally the END_TRANSFER command completion handler will clear the
DWC3_EP_TRANSFER_STARTED flag. However, if the command was sent without
interrupt on completion, then the flag will not be cleared. Make sure to
clear the flag in this case.

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