iio: adc: ad9467: fix ad9434 vref mask

iio: adc: ad9467: fix ad9434 vref mask

The mask setting is 5 bits wide for the ad9434
(ref. data sheet register 0x18 FLEX_VREF). Apparently the settings
from ad9265 were copied by mistake when support for the device was added
to the driver.

Fixes: 4606d0f4b05f ("iio: adc: ad9467: add support for AD9434 high-speed ADC")
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Tomas Melin <tomas.melin@vaisala.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>