hwmon: (ltc4261) set data->valid to 0 if error

If there is an error it is better to set data->valid to 0
so the next call to ltc4261_update_device will always be
executed.

Signed-off-by: Frans Meulenbroeks <fransmeulenbroeks@gmail.com>
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>