greybus: svc: create bidirectional routes on hotplug request

Upon receiving a hotplug request, we need to prepare the routing table
to allow packets to flow between the AP interface and the newly detected
interface.

Tested-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Perry Hung <perry@leaflabs.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>