usb: dwc3: initialize ep0_usb_req.request.dma to avoid kernel panic

It seems possibility that usb_gadget_map_request may not been called
during usb cable connected so ep0_usb_req.request.dma is still by
default 0 which will pass the if condition in unmapping, hence unmap
a request which never been mapped.

Initialize ep0_usb_req.request.dma to DMA_ERROR_CODE will avoid this
corner case kernel panic.

Change-Id: Icf63dc383304c83fe36fb908226240e139a7f9fc
Signed-off-by: Ziqi Chen <ziqic@codeaurora.org>