iio: adc: ad9467: support write/read offset via _calibbias

iio: adc: ad9467: support write/read offset via _calibbias

Support configuring output calibration value. Among the devices
currently supported by this driver, this setting is specific to
ad9434. The offset can be used to calibrate the output against
a known input. The register is called offset, but the procedure
is best mapped internally with calibbias operation.

Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Tomas Melin <tomas.melin@vaisala.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>