sectionsd: try to improve "good time" detection logic

Origin commit data
------------------
Commit: 70e0543382
Author: Stefan Seyfried <seife@tuxbox-git.slipkontur.de>
Date: 2016-01-17 (Sun, 17 Jan 2016)

src/eitd/sectionsd.cpp

@@ -1503,7 +1503,8 @@ void CTimeThread::run()
 					dvb_time = st.getTime();
 					success = true;
 				}
-			}
+			} else
+				retry = false; /* reset bogon detector after invalid read() */
 		}
 		/* default sleep time */
 		sleep_time = ntprefresh * 60;