staging:iio:dds:ad9834 unwind use of is_visible for attrs.

Trivial usecase in which just having two different attr
groups covers all options.  Slightly more code, but a simpler
to follow result.

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>