mfd/bus: sunxi-rsb: Make .remove() callback return void

The driver core ignores the return value of struct device_driver::remove
because there is only little that can be done. To simplify the quest to
make this function return void, let struct sunxi_rsb_driver::remove
return void, too. All users already unconditionally return 0, this
commit makes this obvious and ensures future users don't behave
differently. To simplify even further, make axp20x_device_remove()
return void instead of returning 0 unconditionally, too.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>