hwmon: (ltc2992) Avoid division by zero

Do not allow setting shunt resistor to 0. This results in a division by
zero when performing current value computations based on input voltages
and connected resistor values.

Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Link: https://lore.kernel.org/r/20231011135754.13508-1-antoniu.miclaus@analog.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>