powerpc/fsl_msi: Use chip_data not handler_data

handler_data should be reserved for flow handlers on the dependent
irq, not consumed by the parent irq code that is part of the irq_chip
code.  The msi_data pointer was already set in msidesc->irqhost->hostdata
and being copied to irq_data->chipdata in the msidesc->irqhost->map()
method called via create_irq_mapping, so we can obtain the pointer
from there and free the instance it in teardown_msi_irqs.

Also remove the unnecessary cast of irq_get_handler_data in the
cascade handler, which is the demux flow handler of the parent
msi interrupt.  (This is the expected usage for handler_data).

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>