[SCSI] aic7xxx: fix U160 mode

The new period/dt setting routines don't get the coupling of these
parameters correct.  This means that Domain Validation never gets DT
set, and thus the drive gets restricted to U80.

Fix this by restoring the couplings in the set routines.

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