target: Make pscsi_create_virtdevice use ERR_CAST

This patch changes pscsi_create_virtdevice() to properly return ERR_CAST
instead of a raw pointer upon scsi_host_lookup() failure.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>