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

Add the required SPDX-License-Identifier to the file r8180_93cx6.h,
this clears a 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>