iio: iio.h: fix a warning at the kernel-doc markup

There's a warning at iio.h kernel-doc markup:

	./include/linux/iio/iio.h:644: WARNING: Unknown target name: "devm".

Because it is using {devm_}foo notation. Well, this is not
a valid kernel-doc notation. Also, it prevents creating hyperlinks
to other documentation functions.

So, replace it to a better notation.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/d8f2275c438c459ede4e6fba03ce719cc6ad898b.1599660067.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>