mfd: Fix pcf50633-regulator drvdata usage

Currently the pcf50633-regulator driver data is set to the pcf50633 core
structure, but the pcf50633-regulator remove handler assumes that it is set to
the regulator device. This patch fixes the issue by accessing the pcf506533
core structure through its parent device and setting the driver data to the
regulator device.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>