greybus: connection: make cport lookup thread-safe

Use the gb_connection_mutex when making sure a remote CPort is
available.

This is needed when moving to driver-managed connections that can be
created and destroyed at any time.

Also check for duplicate bundle-less connections.

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