greybus: svc: remove interface->svc pointer

Its not updated/used anymore, remove it. Also move back the struct
gb_svc to svc.c as its not referenced by external users anymore.

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