usb: gadget: uvc: use explicit type instead of void *

The first parameter of __uvcg_iter_strm_cls() is always used in
the context of struct uvcg_streaming_header, so change the
function prototype accordingly.

Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>