iio: ad5064: Fix ad5629/ad5669 shift

The ad5629/ad5669 are the I2C variant of the ad5628/ad5668, which has a SPI
interface. They are mostly identical with the exception that the shift
factor is different. Currently the driver does not take care of this
difference which leads to incorrect DAC output values.

Fix this by introducing a custom channel spec for the ad5629/ad5669 with
the correct shift factor.

Fixes: commit 6a17a076 ("iio:dac:ad5064: Add support for the ad5629r and ad5669r")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>