[SCSI] SPI DV: be more conservative about echo buffer usage

Some SCSI devices apparently get very confused if we try to use the
echo buffer on a non-DT negotiated bus (this mirrors the problems of
using PPR on non-LVD for some devices).  The fix is to be far more
conservative about when we use an echo buffer.  With this patch, we'll
now see what parameters are negotiated by the read only test, and only
look for an echo buffer if DT is negotiated.

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