LinuxKPI: PCI: implement support for more than 1 MSI vector

Following e9715b1c4474333ff119aba3a9a74bff91f72372 and
4b56afaf7bf4fa37bae5b26fd93ee1ff5969c1bb, implement support
for up-to 32 MSI vectors. This is used by wireless drivers.
This also switches msi_desc to an array in order to store
per-vector information.

Sponsored by: The FreeBSD Foundation
Discussed with: grehan (in Dec)
MFC after: 3 days
Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D38222