driver:base:core: Adding a "Return:" line in comment for device_link_add()

The original document doesn't explain the return value directly which
leads to confusing in error checking.

You can find the reason here:
Link: https://lore.kernel.org/all/1d4c39e109bcf288d5900670e024a315.sboyd@kernel.org/Signed-off-by: Yuesong Li <liyuesong@vivo.com>
Link: https://lore.kernel.org/r/20240821040432.4049183-1-liyuesong@vivo.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/base/core.c

@@ -673,6 +673,9 @@ postcore_initcall(devlink_class_init);
  * @supplier: Supplier end of the link.
  * @flags: Link flags.
  *
+ * Return: On success, a device_link struct will be returned.
+ *         On error or invalid flag settings, NULL will be returned.
+ *
  * The caller is responsible for the proper synchronization of the link creation
  * with runtime PM.  First, setting the DL_FLAG_PM_RUNTIME flag will cause the
  * runtime PM framework to take the link into account.  Second, if the