ec_host_cmd: introduce Host Command state

Introduce a current state of Host Command subsystem.

It makes sures that a backend has been initilized and doesn't allow
sending a response twice. There is a possibility, that a command handler
that calls ec_host_cmd_send_response function returns anyway (which is a
mistake).

Signed-off-by: Dawid Niedzwiecki <dawidn@google.com>