usb: gadget: uvc: remove references to DMA_ADDR_INVALID

gadget drivers shouldn't touch req->dma at all,
since UDC drivers are the ones required to handle
mapping and unmapping of the request buffer.

Remove references to DMA_ADDR_INVALID so we don't
creat false expectations to gadget driver writers.

Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>