staging: wlan-ng: fix SPDX comment style in headers

Several of the wlan-ng header files had C++-style SPDX comments. Fixed
checkpatch warnings by replacing with C-style comments, as per the
kernel docs.

Signed-off-by: Tim Collier <osdevtc@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>