ahc(4)/ahd(4): fix target mode on ARM

One of the comments in ahc_execute_scb() notes that the CAM direction is
actually w.r.t. the initiator. As a consequence, all of our sync ops
end up being wrong because the direction is flipped from that of the
transfer. Fix it to do proper invalidation and avoid spewing random
garbage out on the SCSI bus.

Reported and tested by: HP van Braam <hp@tmm.cx>
Reviewed by: imp, mav
Differential Revision: https://reviews.freebsd.org/D44860