Staging: IIO: dds.h convenience macros

Changes since RFC/v1:
IIO: Apply list review feedback

Apply list review feedback:
	Rename attributes to fit IIO convention used in other drivers.
	Provide ddsX_out_enable as opposed to ddsX_out_disable.
	Fix typos.

Changes since v2:
IIO: dds.h: Apply more list review feedback

	Fix mixture of X and [n] notation for indices.
	Remove duplicated descriptions.
	Add store methods to some dds convenience macros.

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