target/iscsi: Fix op=disable + error handling cases in np_store_iser

Writing 0 when iser was not previously enabled, so succeed but do
nothing so that user-space code doesn't need a try: catch block
when ib_isert logic is not available.

Also, return actual error from add_network_portal using PTR_ERR
during op=enable failure.

Signed-off-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>