iio: adis16400: Report pressure channel scale

commit 69ca2d771e4e709c5ae1125858e1246e77ef8b86 upstream.

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")
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>