iio: adc: rtq6056: Correct the sign bit index

iio: adc: rtq6056: Correct the sign bit index

The vshunt/current reported register is a signed 16bit integer. The
sign bit index should be '15', not '16'.

Fixes: 4396f45d211b ("iio: adc: Add rtq6056 support")
Reported-by: Andy Hsu <andy_ya_hsu@wiwynn.com>
Signed-off-by: ChiYuan Huang <cy_huang@richtek.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>