osmo_e1f: Allow to transmit AIS

AIS is an all-1 bit-pattern which is used by intermediate equipment
to indicate that the link somewhere upstream is lost.

Change-Id: Icc5a16ee44ecebe802c4cca0b6cc763a88b6cc05
diff --git a/software/obsolete/osmo_e1f.c b/software/obsolete/osmo_e1f.c
index 2c3fcfb..fa64e17 100644
--- a/software/obsolete/osmo_e1f.c
+++ b/software/obsolete/osmo_e1f.c
@@ -292,6 +292,11 @@
 {
 	int i;
 
+	if (e1i->tx.ais) {
+		memset(out_frame, 0xff, 32);
+		return 0;
+	}
+
 	/* generate TS0 */
 	out_frame[0] = e1_pull_ts0(e1i);