ata_xpt: Use the correct union member when accessing valid

[brooks]

Summary

In principle this should not matter as it's a union and they point to
the same memory location but based on the code above we should be
accessing .sata and not .ata

[brooks]

I found this in our CheriBSD diffs. I'm not sure how @arichardson ran into it.

[arichardson]

I'm not sure why I made that change. I just remember that we had some compiler bugs in that function that went away when I added a printf. I think this was just one of the suspicious things that I saw so I made the change hoping that it would fix the miscompilation.