ASoC: MT6660: make spdxcheck.py happy

The SPDX-License-Identifier shall not be suffixed with anything further.
This makes ./scripts/spdxcheck.py complain:

  sound/soc/codecs/mt6660.c: 1:36 Invalid token: //

Clean up SPDX-License-Identifier line to make spdxcheck.py happy.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>