usb: dwc3: gadget: prevent dwc3_request from being queued twice

Queueing the same request twice can introduce hard-to-debug
problems. At least one function driver - Android's f_mtp.c - is known
to cause this problem.

While that function is out-of-tree, this is a problem that's easy
enough to avoid.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>