staging:iio:dac:ad5791: Convert to extended channel attributes

Use extended channel attributes instead of raw sysfs files for the additional
channel attributes. This allows us to remove some boilerplate code.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>