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

Add SPDX-License-Identifier tag to the start of header file. 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>