iio: lp8788_adc: fix a parent device in _probe()

 The lp8788-adc 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 of
 iio device and device kernel message.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>