usb: dwc3: gadget: remove DWC3_EP_END_TRANSFER_PENDING

Now that we modified the code to fix a race condition, it's clear that
DWC3_EP_END_TRANSFER_PENDING is unnecessary, considering that
DWC3_EP_TRANSFER_STARTED will remain set until End Transfer completes.

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