target: Alter core_pr_dump_initiator_port for ease of use

We use this function exclusively in debug prints. Instead of returning
0 or 1 if isid is present, just set buf to "" if it isn't there. This
saves callers from having to check the return value.

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