genirq/msi: Consolidate MSI descriptor data

All non PCI/MSI usage variants have data structures in struct msi_desc with
only one member: xxx_index. PCI/MSI has a entry_nr member.

Add a common msi_index member to struct msi_desc so all implementations can
share it which allows further consolidation.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Michael Kelley <mikelley@microsoft.com>
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/20211210221814.350967317@linutronix.de