IuUP hack: make RTP patching less general

We currently still patch over an RTP message to make it look like an IuUP
Initialization Ack specifically for the ip.access nano3G femto cell.

Be more specific about it:

- only patch over RTP in 'loopback' mode. osmo-msc specifically leaves the
  endpoint in loopback mode for this hack, so if we're not in 'loopback', then
  this hack is out of place.

- only patch over RTP if the header indicates an IuUP Initialization (check for
  0xe4 byte).

Change-Id: Ia9ec4debc138b34f6ca6a871a8778eafa6c0ba21
1 file changed