hwmon: (ads2828) Only keep data in device data structure if needed

The variables diff_input, ext_vref, and vref_mv are only used in the probe
function and therefore don't need to be kept in the device data structure.

Reviewed-and-Tested-by: Robert Rosengren <robert.rosengren@axis.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>