staging:iio: move iio data return types into types.h for use by inkern

In kernel interfaces need these, so make them available.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>