bus: fsl-mc-msi: Use msi_desc::msi_index

Use the common msi_index member and get rid of the pointless wrapper struct.

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/20211210221814.477386185@linutronix.de