staging: iio: dds: ad9832: Fix kconfig description

The AD9832 driver doesn't support the AD9833 and AD9834

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>