rsl: Properly initialize the cm structure with memset

abis_rsl.c:332:23: warning: 'memset' call operates on objects of type 'struct rsl_ie_chan_mode'
      while the size is based on a different type 'struct rsl_ie_chan_mode *'
      [-Wsizeof-pointer-memaccess]
        memset(cm, 0, sizeof(cm));
               ~~            ^~
abis_rsl.c:332:23: note: did you mean to dereference the argument to 'sizeof' (and multiply it
      by the number of elements)?
        memset(cm, 0, sizeof(cm));
                             ^~
1 warning generated.
diff --git a/openbsc/src/libbsc/abis_rsl.c b/openbsc/src/libbsc/abis_rsl.c
index 6b62656..644706f 100644
--- a/openbsc/src/libbsc/abis_rsl.c
+++ b/openbsc/src/libbsc/abis_rsl.c
@@ -329,7 +329,7 @@
 static int channel_mode_from_lchan(struct rsl_ie_chan_mode *cm,
 				   struct gsm_lchan *lchan)
 {
-	memset(cm, 0, sizeof(cm));
+	memset(cm, 0, sizeof(*cm));
 
 	/* FIXME: what to do with data calls ? */
 	if (lchan->ts->trx->bts->network->dtx_enabled)