mfd: cros_ec_dev: Register shutdown function for debugfs

Reboot or shutdown during delayed works could corrupt communication with
EC and certain I2C controller may not be able to recover from the error
state.

This patch registers a shutdown callback used to cancel the debugfs log
worker thread.

Signed-off-by: Daniel Hung-yu Wu <hywu@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>