hwmon: (lm63) Add sensor type attribute for external sensor on LM96163

On LM96163, the external temperature sensor type is configurable to
either a thermal diode or a 3904 transistor. The chip reports a wrong
temperature if misconfigured. Add writable attribute to support it.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Jean Delvare <khali@linux-fr.org>