rx51_battery: Remove unnecessary platform_set_drvdata()

The driver core clears the driver data to NULL after device_release or on
probe failure, since commit 0998d063
(device-core: Ensure drvdata = NULL when no driver is bound). Thus, it is
not needed to manually clear the device driver data to NULL.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>