mfd: tc6393xb: Make disable callback return void

All implementations return 0, so simplify accordingly.

This is a preparation for making platform remove callbacks return void.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20220619082655.53728-1-u.kleine-koenig@pengutronix.de