iio: dummy: Add correct tabs and spaces to Kconfig

Kconfig from iio/dummy does not follow the coding style recommendations.
According to the coding-style, Lines under a config definition are
indented with one tab, while help text is indented an additional two
spaces. This patch adds the proper tabulation and space.
Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Reviewed-by: Daniel Baluta <daniel.baluta@nxp.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>