siox: Make remove callback return void

The driver core ignores the return value of the remove callback, so
don't give siox drivers the chance to provide a value.

All siox drivers only allocate devm-managed resources in
.probe, so there is no .remove callback to fix.

Tested-by: Thorsten Scherer <t.scherer@eckelmann.de>
Acked-by: Thorsten Scherer <t.scherer@eckelmann.de>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20201125093106.240643-3-u.kleine-koenig@pengutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>