PM / docs: Fix structure references in device.rst

There is a better way to represent structure references than it was
done in device.rst by commit 730c4c05 (PM / sleep / docs: Convert
PM notifiers document to reST), which is to use "struct name" as a
link caption (e.g. :c:type:`struct device <device>`).  That will
cause sphinx to generate a proper reference to the data type in
question (struct device in the example above) and "struct name"
will work as the link in the HTML output.

Fix device.rst by using that convention where applicable.

Fixes: 730c4c05

 (PM / sleep / docs: Convert PM notifiers document to reST)
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>