greybus: connection: prevent drivers from specifying core flags

Prevent drivers from specifying core flags (currently only
GB_CONNECTION_FLAG_CONTROL) when creating connections.

Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>