mei: return error on notification request to a disconnected client

Request for a notification from a disconnected client will be ignored
silently by the FW but the caller should know that the operation hasn't
succeeded.

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