Fix use of busdma(9) KPI in ahci(4).

Use BUS_DMA_NOWAIT for loads at initialization time.
Report actual numeric error code if any problem occurs at the
initialization.

Reported and tested by: pho
Reviewed by: mav
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D18741