Use the standard method for localizing of MSI-X table bar.

Current way, hardcoded value plus heuristic is not conform to the PCI(e)
specification and it fails on systems where MSI-X bar is not initialized by
BIOS/ACPI (many arm or arm64 systems for example).
Instead, use the standard PCI(e) capability for determining of
MSIX table bar address.

MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D27265