of: overlay: Fix kerneldoc warning in of_overlay_remove()

'*ovcs_id' causes a warning because '*' is treated as bold markup:

Documentation/devicetree/kernel-api:56: ../drivers/of/overlay.c:1184: WARNING: Inline emphasis start-string without end-string.

Fix this by using the normal '@' markup for function parameters. That
appears to be enough to keep the '*' from being interpretted as markup.
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Frank Rowand <frowand.list@gmail.com>
Link: https://lore.kernel.org/r/20210421154548.1192903-1-robh@kernel.org/Signed-off-by: Rob Herring <robh@kernel.org>

drivers/of/overlay.c

@@ -1181,7 +1181,7 @@ static int overlay_removal_is_ok(struct overlay_changeset *remove_ovcs)
  * If an error is returned by an overlay changeset post-remove notifier
  * then no further overlay changeset post-remove notifier will be called.
  *
- * Return: 0 on success, or a negative error number.  *ovcs_id is set to
+ * Return: 0 on success, or a negative error number.  *@ovcs_id is set to
  * zero after reverting the changeset, even if a subsequent error occurs.
  */
 int of_overlay_remove(int *ovcs_id)