powerpc/fsl_msi: Don't abuse platform_data for driver_data

The msi platform device driver was abusing dev.platform_data for its
platform_driver_data.  Use the correct pointer for storage.

Platform_data is supposed to be for platforms to communicate to drivers
parameters that are not otherwise discoverable.  Its lifetime matches
the platform_device not the platform device driver.  It is generally
not needed for drivers that only support systems with device trees.

Signed-off-by: Milton Miller <miltonm@bga.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>