power: supply: max8998-charger: Fix platform data retrieval

Since the max8998 MFD driver supports instantiation by DT, platform data
retrieval is handled in MFD probe and cell drivers should get use
the pdata field of max8998_dev struct to obtain them.

Fixes: ee999fb3

 ("mfd: max8998: Add support for Device Tree")
Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
Signed-off-by: Paweł Chmiel <pawel.mikolaj.chmiel@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>