Fix regression in AHCI controller settings.

When the AHCI code was reworked to use FreeBSD struct
definitions, the valid element was mis-transcribed resulting
in the UMDA capability being hidden. This prevented Illumos
from using AHCI disk/cdrom drives.

Fix by using definitions that match the code pre-rework.

PR: 250924
Submitted by: Rolf Stalder
Reported by: Rolf Stalder
MFC after: 3 days