iscsi-target: fix error path in iscsi_target_init_module()

In iscsi_target_init_module() unwind transport protocol registration in case
that iscsit_load_discovery_tpg() failed.

Signed-off-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>