usb: gadget: uvc: Set the V4L2 buffer field to V4L2_FIELD_NONE

The UVC gadget driver doesn't support interlaced video but left the
buffer field uninitialized. Set it to V4L2_FIELD_NONE.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>