mfd: asic3: Make asic3_gpio_remove() return void

Up to now asic3_gpio_remove() returns zero unconditionally. This makes it
easier to see in the caller that there is no error to handle.

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

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/20220530192430.2108217-2-u.kleine-koenig@pengutronix.de