greybus: interface: reduce control-device lifetime

Make the control-device lifetime coincide with when the interface is
enabled (enumerated).

This is needed to be able register a new control device after a mode
switch.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>