greybus: interface: clean up control-connection handling

Clean up control-connection handling by managing it through the control
structure and a higher-level control interface. Also make both the
control structure and connection lifetimes coincide with that of the
interface.

The control connection is now only enabled and disabled when the
interface is initialised and removed.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>