staging: vt6656: Use the correct style for SPDX license Identifier

This patch corrects the style for SPDX license Identifier in mac.h
by using "/* */" in place of "//" as per Linux kernel licensing rules.
Issue found by checkpatch.

Signed-off-by: Nishad Kamdar <nishadkamdar@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>