libmsc/gsm_09_11.c: properly handle MS-initiated release

According to GSM TS 02.90, section 4.3, release of the connection
used for SS/USSD is normally the responsibility of the network.
But the user may also initiate connection release, e.g. by
pressing the 'red button'.

TTCN-3 test case: I7936ed5072ed2ae02f039dc90a1fece1e7f70a70
Change-Id: I76fc277bf9db614a97824b1541cd5bb75aa3e29d
1 file changed