usb: dwc3: gadget: cope with XferNotReady before usb_ep_queue()

If XferNotReady comes before usb_ep_queue() we will
set our PENDING request flag and wait for a
request. However, originally, we were assuming
usb_ep_queue() would always happen before our first
XferNotReady and that causes a corner case where we
could try to issue ENDTRANSFER command before
STARTTRANSFER.

Let's fix that by tracking endpoints which have been
started.

Reported-by: Janusz Dziedzic <januszx.dziedzic@intel.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>