usb: dwc3: gadget: only unmap requests from DMA if mapped

A recent optimization was made so that a request put on the
pending_list wouldn't get mapped for DMA until just before
preparing a TRB for it. However, this poses a problem in case
the request is dequeued or the endpoint is disabled before the
mapping is done as that would lead to dwc3_gadget_giveback()
unconditionally calling usb_gadget_unmap_request_for_dev() with
an invalid request->dma handle. Depending on the platform's DMA
implementation the unmap operation could result in a panic.

Since we know a successful mapping is a prerequisite for getting
a TRB, the unmap can be conditionally called only when req->trb
is non-NULL.

Fixes: cdb55b39 ("usb: dwc3: gadget: lazily map requests for DMA")
Signed-off-by: Jack Pham <jackp@codeaurora.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>