Changes

mgw: Simplify and redo code around ssrc patch feature

The previous logic to configure SSRC patching was over complex and
confusing, with even some broken logic like "patch only once", which is
not really needed. Hence, simplify the internal logic to either patch
SSRC always or don't do it, based on VTY configuration.

The "force SSRC" feature was added in
db2d431697609d473de433b7028f81ce499a02c0 and further changed in
e2292f3aa1e9ae1faf5521a9cb108775f1ac0540, due to the need for nanoBTS to
always keep the same SSRC on received packets. However, it makes no
sense that is actually needed only once: if remote end changed several
times, then we should keep patching in that case.

This change allows getting rid of confusing applying of settings at a
later time through mgcp_rtp_end_config(), which is no longer needed and
can be dropped, simplifying steps during CRCX/MDCX/DLCX.

Change-Id: Ib7216a775f4ad126e62b9e99aff381fd45015819