osmux: Use DUMMY ft msg as per Osmux spec

That MGCP_DUMMY_LOAD is an old hack prior to Osmux spec update, and it's
not nice since it cannot be 100% distinguished from a usual AMR ft
frame.

Let's use the correct DUMMY ft type and build it according spec. Allow
handling differently the old format for a while until we are sure no old
implementations (like bsc-nat) exist sending that kind of message.

Change-Id: Ib17d20b87b28aade49ba60519b56a96e694819af
1 file changed