a_iface: Centralize/wrap BSSAP / N-DATA transmission

We don't want multiple callers to osmo_sccp_tx_data_msg() each having
to hex-dump a log message about the to-be-transmitted message, with
half of the caller sitest missing that printing.  Let's centralize
all calls of osmo_sccp_tx_data_msg() in a wrapper function which
takes care of the related OSMO_ASSERT() and the related printing.

Change-Id: I6159ea72cc8e0650eda6c49544acd65e9c15e817
diff --git a/tests/msc_vlr/msc_vlr_test_ms_timeout.err b/tests/msc_vlr/msc_vlr_test_ms_timeout.err
index e09389c..72d116a 100644
--- a/tests/msc_vlr/msc_vlr_test_ms_timeout.err
+++ b/tests/msc_vlr/msc_vlr_test_ms_timeout.err
@@ -686,6 +686,7 @@
 DVLR vlr_lu_fsm(IMSI-901700000004620:GERAN-A-0:LU){VLR_ULA_S_WAIT_AUTH}: Set Ciphering Mode
 DMM IMSI-901700000004620: to determine whether A5/3 is supported, first ask for a Classmark Update to obtain Classmark 2
 DBSSAP RAN_conn(IMSI-901700000004620:GERAN-A-0:LU){RAN_CONN_S_AUTH_CIPH}: Tx BSSMAP CLASSMARK REQUEST to BSC
+DBSSAP RAN_conn(IMSI-901700000004620:GERAN-A-0:LU){RAN_CONN_S_AUTH_CIPH}: N-DATA.req([])
   BSC <--BSSAP-BSS-MANAGEMENT-- MSC: CLASSMARK REQ [L3]> 00 01 58 
 DMM RAN_conn(IMSI-901700000004620:GERAN-A-0:LU){RAN_CONN_S_AUTH_CIPH}: state_chg to RAN_CONN_S_WAIT_CLASSMARK_UPDATE
 DVLR vlr_lu_fsm(IMSI-901700000004620:GERAN-A-0:LU){VLR_ULA_S_WAIT_AUTH}: state_chg to VLR_ULA_S_WAIT_CIPH