staging:iio:dac:ad5446 unwind use of is_visible for attrs.

Trivial case where no attributes are valid for some parts. Better
handled using two iio_info structures and selecting the right one
at probe time.

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