greybus: connection: fix potential null-deref when binding protocol

We can have connections without bundles so we must not use the bundle
device for error messages when failing to look up a protocol.

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>