This means we'll start correcting frequency ~5 minutes after start,
not ~3.5 ones.
With previos settings I still often see largish ~0.7s initial offsets
only about 1/2 corrected before frequency correction kicks in,
resulting in ~200ppm "correction" which is then slowly undone.
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>