db.c: Move the schema revision check after creating the tables

When starting the first time there are no tables, doing a revision
check will fail and bsc_hack will exit without tables created. Do
the revision check within db_prepare and allow new tables to be
created before.
diff --git a/openbsc/src/db.c b/openbsc/src/db.c
index 90bee69..9ad39d3 100644
--- a/openbsc/src/db.c
+++ b/openbsc/src/db.c
@@ -161,12 +161,6 @@
 	if (dbi_conn_connect(conn) < 0)
 		goto out_err;
 
-	if (check_db_revision() < 0) {
-		fprintf(stderr, "Database schema revision invalid, "
-			"please update your database schema\n");
-		goto out_err;
-	}
-
 	return 0;
 
 out_err:
@@ -190,6 +184,12 @@
 		dbi_result_free(result);
 	}
 
+	if (check_db_revision() < 0) {
+		fprintf(stderr, "Database schema revision invalid, "
+			"please update your database schema\n");
+                return -1;
+	}
+
 	return 0;
 }