greybus: connection: separate connection creation and enabling

Separate connection creation from enabling.

This will ultimately allow connection structures to be created while
parsing manifests, but the connections to not be enabled until a driver
is bound.

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>