gpio: pcf857x: Make teardown callback return void

All teardown functions return 0. Also there is little sense in returning
a negative error code from an i2c remove function as this only results in
emitting an error message but the device is removed nevertheless.

This patch is a preparation for making i2c remove callbacks return void.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>