usb: gadget: uvc: move structs to common header

usb: gadget: uvc: move structs to common header

The functions and structs of the configfs interface should also be used
by the uvc gadget driver. This patch prepares the stack by moving the
common structs and functions to the common header file.

Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Link: https://lore.kernel.org/r/20220421211427.3400834-5-m.grzeschik@pengutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>