[SCSI] aic94xx: Lock DDB read/write accesses

Extend the use of the DDB lock to include all DDB accesses, because
DDB updates now occur from multiple threads.  This fixes the SMP timeout
problems that we were occasionally seeing with a x260, because the
controller got confused when the DDBs got corrupted.

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