staging:rtl8192u: Add missing SPDX-License-Identifier - Style

Add the missing SPDX-License-Identifier tag to file to clear the
checkpatch issue.

This is a coding style change which should have no impact on runtime
code execution.

Signed-off-by: John Whitmore <johnfwhitmore@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>