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

[kevans]

Summary

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.

[hp_tmm.cx]

I have tested this on an Raspberry PI CM4 with the IO board and this works there.

[mav]

Looks good to me, but if you wish, couple cosmetic thoughts.

sys/dev/aic7xxx/aic7xxx_osm.c

306

post asks to be bool, but considering age of the driver...

309

flags in struct ccb_hdr is uint32_t.