staging: rtlwifi: Replace license boilerplate with SPDX identifiers

This satisfies a checkpatch.pl warning and is the preferred method for
notating the license due to its lack of ambiguity.

Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>