ix, ixv: Read msix_bar from device configuration
Instead of predicting the MSI-X bar index based on the device's MAC
type, read it from the device's PCI configuration instead.
Submitted by: Piotr Pietruszewski <firstname.lastname@example.org>
Reviewed by: erj@
MFC after: 3 days
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D21547