dt-bindings: net: btusb: change reference file name

Due to usb-device.txt is converted into usb-device.yaml,
so modify reference file names at the same time.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/20201225075258.33352-2-chunfeng.yun@mediatek.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>