[SCSI] aic94xx: Fix DDB and SCB initialization

Ed Chim of Adaptec informs us that the DDB registers need to be zeroed at
initialization time and that some SCB initializations need to happen even if
we don't use the SCB.

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>