iio: adis16400: Report pressure channel scale

Add the scale for the pressure channel, which is currently missing.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Fixes: 76ada52f ("iio:adis16400: Add support for the adis16448")
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>