staging:iio:adc:ad7606 unwind use of is_visible for attrs.

This is the most controversial of this set of is_visible removals.
There are two conditions controlling availability of attrs resulting
in 4 different attribute groups.

Still for a few more lines things are clearer to read to my mind.

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>