cosmetic: rename gsm_subscriber_connection->conn_fsm to ->fi

Match osmo-bsc's naming of the subscriber connection's FSM instance; 'conn->fi'
makes more sense anyway than 'conn->conn_fsm'.

BTW, an upcoming commit will do away with the legacy from libbsc/libmsc duality
and firmly glue the conn allocation to the fi.

Related: OS#3122
Change-Id: If442f2ba78d9722b1065ec30c9a13f372b6a8caa
15 files changed