sunxi: gpio: Return void from setter functions

The return values of these functions are always zero, and they are
never checked. Since they are not needed, remove them.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>