[SCSI] aic7xxx: pause sequencer before touching SBLKCTL

Some cards need to pause the sequencer before the SBLKCTL register is
touched.  This fixes a PCI related oops seen on powerpc macs with this
card caused by trying to ascertain the bus signalling before beginning
domain validation.
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>