usb: dwc3: gadget: NULL check for ep descriptor in xfer complete

There is an issue where dwc3_endpoint_transfer_complete() is called
after the endpoint is disabled, which causes a NULL pointer dereference.
So to fix, this add a NULL check for endpoint descriptor.

CRs-fixed: 724651
Change-Id: Ic6844ea104d7810457cfa01473fcfebb21251357
Signed-off-by: Azhar Shaikh <azhars@codeaurora.org>