Allow byte reads of AHCI registers.

This is needed to support Windows guests that use byte reads to access certain
AHCI registers (e.g. PxTFD.Status and PxTFD.Error).

Reviewed by: grehan, mav
Reported by: Leon Dang (ldang@nahannisys.com)
Differential Revision: https://reviews.freebsd.org/D2469
MFC after: 2 weeks