usb: gadget: uvc: Move userspace API definition to public header

The UVC gadget userspace API (V4L2 events and custom ioctls) is defined
in a header internal to the kernel. Move it to a new public header to
make it accessible to userspace.

The UVC_INTF_CONTROL and UVC_INTF_STREAMING macros are not used, so
remove them in the process.

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