USB: dwc3: gadget: Don't queue endless req through generic ep_queue

For DBM endpoints, queueing endless request through generic ep_queue
API should not allowed. Otherwise it might lead to crash when called
dwc3_gadget_map_request(). Hence add check whether it is DBM endpoint
or not and if it is, return error.

Change-Id: I7923df35f30293ac7ebb94b656f709f4ba5e00e5
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>