ahc/ahd: Fix target mode operation

After 5e63cdb457f9 the drivers didn't clear CAM_DIS_DISCONNECT in
ah*_handle_target_cmd() when needed, only set it.

Reported/tested by: HP van Braam <hp@tmm.cx>
MFC after: 1 week

(cherry picked from commit f1e4c095777db088645008848b3bfb87146c13ec)