target/pscsi: Fix NULL pointer dereference in get_device_type

This patch fixes a NULL pointer dereference OOPs with pSCSI backends
within target_core_stat.c code.  The bug is caused by a configfs attr
read if no pscsi_dev_virt->pdv_sd has been configured.

Reported-by: Olaf Hering <olaf@aepfle.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>