hwmon: (pmbus/ltc2978) Fix peak attribute handling

commit dbd712c2 upstream.

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>
Acked-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>