iio: adc: ina2xx: Shift bus voltage register to mask flag bits

Lower bits of the INA219/220 bus voltage register are conversion
status flags, properly shift the value.

When reading via IIO buffer, the value is passed on unaltered,
shifting is the responsibility of the user.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>