sgsn: use sccp_simple_client API to setup ss7 id 0 instead of 1

Implementation of osmo_sccp_simple_client() API internally uses ss7 id
1, which is confusing since there's no 0 in use in osmo-sgsn. Let's
explicitly use the 0 one so it is configured by "cs7 instance 0" in the
VTY.

Related: OS#4157
Change-Id: I0e23a6a76ebcba0b1b424e3d3b20d06c1da44cbe