driver core: Add missing description of new struct device_link field

Commit 36003d4c ("driver core: Fix PM-runtime for links added
during consumer probe") forgot to add a kerneldoc decription for the
new struct device_link member added by it, so do that now.

Fixes: 36003d4c ("driver core: Fix PM-runtime for links added during consumer probe")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>