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

[ Upstream commit cb90a2c6f77fe9b43d1e3f759bb2f13fe7fa1811 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>