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

(cherry picked from commit 9dcf39575efb2ff32f955d9e04e04af28d45d798)