gpio: max730x: Make __max730x_remove() return void

An spi or i2c remove callback is only called for devices that probed
successfully. In this case this implies that __max730x_probe() set a
non-NULL driver data. So the check ts == NULL is never true. With this
check dropped, __max730x_remove() 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: Bartosz Golaszewski <brgl@bgdev.pl>