mei: document that mei_msg_hdr_init returns ERR_PTR

Document that mei_msg_hdr_init returns ERR_PTR.
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Link: https://lore.kernel.org/r/20210129120752.850325-2-tomas.winkler@intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/mei/client.c

@@ -1737,7 +1737,7 @@ static inline u8 mei_ext_hdr_set_vtag(struct mei_ext_hdr *ext, u8 vtag)
  *
  * @cb: message callback structure
  *
- * Return: a pointer to initialized header
+ * Return: a pointer to initialized header or ERR_PTR on failure
  */
 static struct mei_msg_hdr *mei_msg_hdr_init(const struct mei_cl_cb *cb)
 {