s390/pci: Rework MSI descriptor walk

Replace the about to vanish iterators and make use of the filtering.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Niklas Schnelle <schnelle@linux.ibm.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Acked-by: Niklas Schnelle <schnelle@linux.ibm.com>
Link: https://lore.kernel.org/r/20211206210748.305656158@linutronix.de