Changes

vlr: Stop silent call before deduping subscribers

Before this fix, A use count mismatch could be reached by:

* Completing a location updating procedure with TMSI.
* Disconnecting from the BTS.
* Starting a silent call from the MSC.
* Registering again with the same IMSI but a different TMSI.

This would cause the a new subscriber to be created without
the silent call use count, which in turn would cause the
assert in `vlr_subscr_put` in `trans_free` to fail with use count of -1.

Change-Id: If23f8b0e42d4a3a8bf1c8f5ca81b045834b6cccd