PCI/MSI: Use msi_add_msi_desc()

Simplify the allocation of MSI descriptors by using msi_add_msi_desc()
which moves the storage handling to core code and prepares for dynamic
extension of the MSI-X vector space.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Michael Kelley <mikelley@microsoft.com>
Tested-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://lore.kernel.org/r/20211206210748.035348646@linutronix.de