[SCSI] iscsi: fix nop-in handling

From: zhenyu.z.wang@intel.com

This add check to NOOP_IN's ttt, when it's ~0UL we should not send
NOOP_OUT by spec (plus some cleanup).

Signed-off-by: Alex Aizman <itn780@yahoo.com>
Signed-off-by: Dmitry Yusupov <dmitry_yus@yahoo.com>
Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>