Use callback for polling data

Previously rs->rtp_bfd.when and rs->rtcp_bfd.when value of BSC_FD_READ
was overridden by 0. Fix it.

Reported by coverity: CID 1357763.

Change-Id: I11940cbf50361c4ea262d7001a16f9a982392f27
diff --git a/src/trau/osmo_ortp.c b/src/trau/osmo_ortp.c
index cc1d748..0bb1197 100644
--- a/src/trau/osmo_ortp.c
+++ b/src/trau/osmo_ortp.c
@@ -218,7 +218,6 @@
 	rs->rtp_bfd.fd = rtp_session_get_rtp_socket(rs->sess);
 	rs->rtcp_bfd.fd = rtp_session_get_rtcp_socket(rs->sess);
 	rs->rtp_bfd.when = rs->rtcp_bfd.when = BSC_FD_READ;
-	rs->rtp_bfd.when = rs->rtcp_bfd.when = 0;
 	rs->rtp_bfd.data = rs->rtcp_bfd.data = rs;
 	rs->rtp_bfd.cb = osmo_rtp_fd_cb;
 	rs->rtcp_bfd.cb = osmo_rtcp_fd_cb;