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.

PR: 239704
Submitted by: Piotr Pietruszewski <piotr.pietruszewski@intel.com>
Reviewed by: erj@
MFC after: 3 days
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D21547