nvd: For AHCI attached devices, report ahci bridge

When an NVME device is attached via a AHCI controller, we have no access
to its config space. So instead of information about the nvme drive
itself, return info about the AHCI controller as the next best
thing. Since the Intel Hardware RAID support looks at these values, this
likely is best.

Sponsored by: Netflix
Reviewed by: mav
Differential Revision: https://reviews.freebsd.org/D33286

(cherry picked from commit b8194f37666768dac35a0e1105c41242aad9b2d0)