staging:iio:adc:ad7192 unwind use of is_visible for attribute group.

It saves a couple of lines of code but reduces simplicity of code.
I generally wish to discourage use of is_visible throughout IIO.

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>