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

[ Upstream commit 2e644384 ]

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>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>