usb: gadget: uvc: configfs: Move function to avoid forward declaration

The to_f_uvc_opts() function is forward-declared without needing to, as
its definition can simply be moved up in the file. Fix it.

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