USB: serial: keyspan_usa: add proper SPDX lines for .h files

The keyspan_usa??msg.h files are under a BSD-3 style license, so
properly label them as such with a SPDX line at the top of the file.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>