Following e9715b1c44 and
4b56afaf7b, 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