ix, ixv: Read msix_bar from device configuration

[piotr.pietruszewski_intel.com]

Summary

Instead of predicting MSI-X bar basing on mac type, read it from device
configuration.

This patch resolves PR 239704.

[erj]

Is there a reason that we wouldn't do this for all of our drivers, instead of hardcoding the BAR index?

[jacob.e.keller_intel.com]

I agree with Eric, we should depend on the device to be configured properly, and push to get the images for the device updated if they're wrong.

[jhb]

If the BAR varies across devices, I think using pci_msix_table_bar is probably the simplest thing to do.