power: supply: core: introduce dev_to_psy()

power: supply: core: introduce dev_to_psy()

The psy core and drivers currently use dev_get_drvdata() to go from a
'struct device' to its 'struct power_supply'.
This is not typesafe and or documented.

Introduce a new helper to make this pattern explicit.
Instead of using dev_get_drvdata(), use container_of_const() which
also preserves the constness.
Furthermore 'dev' does need to be dereferenced anymore and at some point
the drvdata could be reused for something else.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/r/20241210-power-supply-dev_to_psy-v2-7-9d8c9d24cfe4@weissschuh.net
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>