platform-msi: Store platform private data pointer in msi_device_data

Storing the platform private data in a MSI descriptor is sloppy at
best. The data belongs to the device and not to the descriptor.
Add a pointer to struct msi_device_data and store the pointer there.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20211210221814.287680528@linutronix.de