docs / driver-api: Fix structure references in device_link.rst

The format of the structure references in device_link.rst is
incorrect, because it doesn't cause proper references to the
struct data types to be generated (for struct dev_pm_domain in
particular).

Fix that by using the :c:type:`struct name <name>` convention
for encoding references to struct data types.

Fixes: aad80040

 (Documentation/core-api/device_link: Add initial documentation)
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>