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.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>