hwmon: (lm63) Add support for writing the external critical temperature

On LM64, the external critical temperature limit is always writable. On LM96163,
it is writable if the chip is configured for it. Add conditional support for
writing the register dependent on chip type and configuration.

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