power: supply: core: use device mutex wrappers

The mutex is an implementation detail of struct device.
Use the dedicated wrappers to access the field.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/r/20241005-power-supply-cleanups-v1-1-45303b2d0a4d@weissschuh.net


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