power: supply: sysfs: use power_supply_property_is_writeable()

Instead of open-coding the helper use it directly.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/r/20240608-power-supply-extensions-v2-1-2dcd35b012ad@weissschuh.net


Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>