iser-target: check device before dereferencing its variable

This patch changes isert_connect_release() to correctly check for
the existence struct isert_device *device before checking for
isert_device->use_frwr.

Signed-off-by: Vu Pham <vu@mellanox.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>