[SCSI] aic79xx, aic7xxx: Fix incorrect width setting

Wide transfers are required for every setting of PPR apart from QAS.
It seems the DV code starts at the minimum, which turns on DT and Wide
regardless of the setting of max_width.  Redo the PPR and period
setting routines to respect max_width (i.e. start at period = 10 if it
is zero).

This fixes bugzilla 8366

Acked-by: "Freels, James D." <freelsjd@ornl.gov>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>