target: use correct sense code for LUN communication failure

The ASC/ASCQ code for 'Logical Unit Communication failure' is
0x08/0x00; 0x80/0x00 is vendor specific.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Cc: Nicholas Bellinger <nab@risingtidesystems.com>
Cc: stable@vger.kernel.org
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>