aic94xx: remove SCSI host before detaching from SAS transport

commit cff549e4

 ("scsi: proper state checking and module refcount
handling in scsi_device_get") the reference count of scsi device was
changed, which could lead to when rmmod with at least on drive attached,
SCSI error handle will run into infinite loop, and lockup the system.

Fix it by remove scsi host first, this way scsi core will not send
commands down after detaching SAS transport.

This is a follow up fix for Benjamin's fix for pm80xx.

See also:
http://www.spinics.net/lists/linux-scsi/msg90088.html

Signed-off-by: Jack Wang <jinpu.wang@profitbricks.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>