bus: fsl-mc: fsl-mc-allocator: Rework MSI handling

Storing a pointer to the MSI descriptor just to track the Linux interrupt
number is daft. Just store the interrupt number and be done with it.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20211210221815.207838579@linutronix.de