LinuxKPI: pci: update struct msi_desc

It seems struct msi_desc is setup differently (or was changed) compared
to how we added it a while ago. Catch up in order to keep drivers
directly accessing fields compiling.

Sponsored by: The FreeBSD Foundation
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D40175

(cherry picked from commit 17bde9cbc6fc3ac219aa11243c0b75025786f628)