usb: gadget: forbid queuing request to a disabled ep

Queue a request to disabled ep  doesn't make sense, and induce caller
make mistakes.

Here is a example for the android mtp gadget function driver. A mem
corruption can happen on below senario.
1) On disconnect, mtp driver disable its EPs,
2) During send_file_work and receive_file_work, mtp queues a request
   to ep. (The mtp driver need improve its synchronization logic!)
3) mtp_function_unbind is invoked and all mtp requests are freed.
4) when udc process the request queued on step 2, will cause kernel
   NULL pointer dereference exception.

Signed-off-by: Du, Changbin <changbin.du@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>