hwmon: (ina209) Handled signed registers

The shunt voltage and current registers are signed 16-bit values so
handle them as such.

Signed-off-by: Joe Schaack <jschaack@xes-inc.com>
Reviewed-by: Aaron Sierra <asierra@xes-inc.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>