Introduce gsm48_lchan2chan_desc_as_configured()

gsm48_lchan2chan_desc_as_configured() is similar to gsm48_lchan2chan_desc(),
but uses the *configured* channel combination, rather than the currently
active one.

Change-Id: Id4043218fb770e8420f19a4ef9428680ecdfd286
Related: OS#3532
diff --git a/include/osmocom/bsc/gsm_data.h b/include/osmocom/bsc/gsm_data.h
index 68f888f..e408ff9 100644
--- a/include/osmocom/bsc/gsm_data.h
+++ b/include/osmocom/bsc/gsm_data.h
@@ -1200,6 +1200,7 @@
 
 void gsm48_lchan2chan_desc(struct gsm48_chan_desc *cd,
 			   const struct gsm_lchan *lchan);
+void gsm48_lchan2chan_desc_as_configured(struct gsm48_chan_desc *cd, const struct gsm_lchan *lchan);
 
 /* return the gsm_lchan for the CBCH (if it exists at all) */
 struct gsm_lchan *gsm_bts_get_cbch(struct gsm_bts *bts);