mei: use the same bus msg for connect and disconnect request

structs hbm_client_connect_request and hbm_client_disconnect_request
have the same layout so we can drop the later

Add kdoc for the request and response structure so it is clear
they can be used for both purposes

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>