rtp_bridge: Demote a chatty ERROR log message to DEBUG level.

Not having a second leg of an MGCP endpoint is a normal situtation
and can't be treated as an ERROR message, especially not as an ERROR
message logged on every RTP packet. This happens routinely at
the beginning of call setup and we get tens of ERROR messages in
the logs for every call.

Change-Id: If741a742208772bda4e59236345d7ae650368d5a
1 file changed