remove code dup: add msc_mgcp_try_call_assignment()

Various places in the code check a flag whether assignment was started and
launch it. To fix incoming-call-during-ongoing-call, I will tweak that logic.
To be able to do that only in one place, remove code dup.

Cosmetic preparation for I1f8746e7babfcd3028a4d2c0ba260c608c686c76 and
I0ba216b737909e92080a722db26e3577726c63cb/

Depends: I11b182a03f5ecb6df7cd8f260757d3626c8e945d (libosmocore: LOGPFSMSL)
Change-Id: I11c0b7dc3f1a747028629b48e522bb3b864884ba
5 files changed