ntp: set maximum number of iburst samples to size of reach register

Explicitly set the number of iburst samples to the size of the register
to make sure there are at least 7 reachability updates and the
initstepslew mode can be ended.

sources.h

@@ -34,6 +34,9 @@
 #include "ntp.h"
 #include "reports.h"
 
+/* Size of the source reachability register */
+#define SOURCE_REACH_BITS 8
+
 /* This datatype is used to hold information about sources.  The
    instance must be passed when calling many of the interface
    functions */