hwmon: (adt7475) Cache limits for 60 seconds

The comment says that limits are cached for 60 seconds but the code
actually caches them for only 2 seconds. Align the code on the
comment, as 60 seconds makes more sense.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Cc: Jordan Crouse <jordan@cosmicpenguin.net>