greybus: svc: save pointer to struct gb_svc in struct gb_interface

Its another special protocol (just like control protocol) and is
required to be accessed from other files, lets save a pointer to it in
interface structure.

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