stex: Add hotplug support

1. Add hotplug support. Pegasus support surprise removal. To this end, I
   use return_abnormal_state function to return DID_NO_CONNECT for all
   commands which sent to driver.

2. Remove stex_hba_stop in stex_remove because we cannot send command to
   device after hotplug.

3. Add new device status: MU_STATE_STOP, MU_STATE_NOCONNECT,
   MU_STATE_STOP. MU_STATE_STOP is currently not referenced.
   MU_STATE_NOCONNECT represent that device is plugged out from the
   host.

4. Use return_abnormal_function() to substitute part of code in
   stex_do_reset.

Signed-off-by: Charles Chiou <charles.chiou@tw.promise.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>