usb: gadget: uvc: Move trace parameter to function module

The trace module parameter controls output of debugging messages in the
UVC function driver. Move it from the webcam module to the UVC function
module where it belongs. This allows ConfigFS-based UVC gadgets to
control tracing.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>