greybus: loopback: use the bundle struct device instead of the connector

We are removing struct device from the gb_connection structure in the
near future.  The gb_bundle structure's struct device should be used as
a replacement.

This patch moves the loopback driver to use the bundle pointer instead
of the connection pointer.

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