[SCSI] aic7xxx: fix bug in DT handing

Basically DT isn't reported or handled at all.  The problem is that
lines of code like this:

spi_dt(starget) = tinfo->curr.ppr_options & MSG_EXT_PPR_DT_REQ;

don't do what you think they do when spi_dt is a single bit variable.

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