iio: adc: spmi-vadc: Fix overflow in output value normalization

With 'dx' equal to 0.625V and 15 bit ADC, calculations overflow
when difference against GND is ~20% of the ADC range. Fix this.

Signed-off-by: Ivan T. Ivanov <ivan.ivanov@linaro.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>