Changes

Release BSS connection when SS message is rejected

When an initial call-independent supplementary service message is
received, it is checked whether the message is a REGISTER message.
If it is not, the MSC will reject it by sending a RELEASE COMPLETE
message.

This patch ensures that the MSC_A use counter is decremented, so
that the BSS connection is released, if it is not used by any other
transaction.

Without this patch, the msc_a_fsm would wait 5 seconds for an initial
transaction before releasing the BSS connection.

Related: OS#6427
Change-Id: Ic6765e5d480735e67d97f0f560da24653b26d487