staging: rtl8712: add SPDX identifiers

This satisfies a checkpatch warning and is the preferred
method for notating the license.

The SPDX identifier is a legally binding shorthand, which
can be used instead of the full boiler plate text.

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>