power_supply: wm97xx_battery: use power_supply_get_drvdata

As the power supply framework provides a way to store and retrieve
private supply data, use it.

In the process, change the platform data for wm97xx_battery from a
container of a single struct wm97xx_batt_pdata to the direct point to wm97xx_batt_pdata.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>