[SCSI] fix aic7xxx performance issues since 2.6.12-rc2

Several people noticed we dropped quite a bit on benchmark figures.
OK, it was my fault but unfortunately I discovered I ran out of brown
paper bags a while ago and forgot to reorder them.

The issue is that a construct introduced in the conversion of the
driver to use the transport class keyed off whether the block request
was tagged or not.  However, the aic7xxx driver doesn't properly set
up the block layer TCQ (it uses the wrong API), so the driver now
things all requests are untagged and we keep it to a queue depth of a
single element.  Oops.

The fix is to use the correct TCQ API.

Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>