usb: gadget: uvc: rename functions to avoid conflicts with host uvc

Prepare for separate compilation of uvc function's components.
Some symbols will have to be exported, so rename to avoid
conflicts with functions of the same name in host uvc.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Tested-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
[Rename uvc_video_pump and uvc_queue_head as well]
[Rename forgotten uvc_queue_cancel instance in a comment]
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>