mfd: cros_ec: cleanup: cros_ec.c is GPL licensed, like all the others.

This module has always been GPL licensed. It was just missing the explicit
declaration to avoid tainting the kernel.

Signed-off-by: Bill Richardson <wfrichar@chromium.org>
Signed-off-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>