legacy-mgcp: switch to new osmux output APIs

Older ones are being deprecated as they may generate interleaved
packets.

This commit is a forward-port of openbsc.git Change-Id
I189564fc63139c15314db8975afd423c7153ea32.

Change-Id: I9b8a19e5b8d62deaa9bbb92d49d99e8c33b7e345
1 file changed