hwmon: (pmbus/ltc2978) Fix peak attribute handling

Peak attributes were not initialized and cleared correctly.
Also, temp2_max is only supported on page 0 and thus does not need to be
an array.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Cc: stable@vger.kernel.org # 3.2+
Acked-by: Jean Delvare <khali@linux-fr.org>