[sccp] Use msgb->l2h instead of msgb->data for CR

Using msgb->data only works as long as msgb->data == msgb->l2h.. In
the case of receiving a MSU unit from a E1 link, or even receiving
the IPA header we will have some non SCCP data at msgb->data and then
cast garbage to what we think is making sense..
Use msgb->l2h and everything is fine.
diff --git a/openbsc/src/sccp/sccp.c b/openbsc/src/sccp/sccp.c
index a199266..94fd017 100644
--- a/openbsc/src/sccp/sccp.c
+++ b/openbsc/src/sccp/sccp.c
@@ -208,7 +208,7 @@
 	static const u_int32_t called_offset =
 			offsetof(struct sccp_connection_request, variable_called);
 
-	struct sccp_connection_request *req = (struct sccp_connection_request *)msgb->data;
+	struct sccp_connection_request *req = (struct sccp_connection_request *)msgb->l2h;
 	struct sccp_optional_data optional_data;
 
 	/* header check */