lp8727_charger: Fix buggy code of NULL pdata

LP8727 platform data is optional, so the driver should work even the
platform data is NULL.

To check the platform data, charging parameter data should be changed to
the pointer type.

Fix NULL point access problem when getting the battery properties. When
the data is NULL, just return as invalid value.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>