ata_xpt: Use the correct union member when accessing valid.

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.

Submitted by: arichardson
Reviewed by: scottl, imp
Obtained from: CheriBSD
MFC after: 1 week
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D21002