mfd: tps65912: Make tps65912_device_exit() return void

Up to now tps65912_device_exit() returns zero unconditionally. Make it
return void instead which makes it easier to see in the callers that
there is no error to handle.

Also the return value of i2c and spi remove callbacks is ignored anyway.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211012153945.2651412-11-u.kleine-koenig@pengutronix.de