staging: iio: dummy: replace block comment with single line comment

Replace one block comment with a single line comment. Follow style of
previous comment and omit word channel, since it's clear the topic is
channels.

Addresses checkpatch.pl:
WARNING: Block comments use * on subsequent lines

Signed-off-by: Alison Schofield <amsfield22@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>