usb: dwc3: gadget: clear DWC3_EP_TRANSFER_STARTED on cmd complete

We must wait until End Transfer completes in order to clear
DWC3_EP_TRANSFER_STARTED, otherwise we may confuse the driver.

This patch is in preparation to fix a rare race condition that happens
upon Disconnect Interrupt.

Tested-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>