lp8788-charger: Fix a parent device in _probe()

The lp8788-charger is a platform driver of lp8788-mfd. The platform device
is allocated when mfd_add_devices() is called in lp8788-mfd. On the other
hand, 'lp->dev' is the i2c client device.

Therefore, this 'platform_device' is a proper parent device in case of
resource managed mem alloc and device kernel message.

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