greybus: svc: store protocol version

Store the "negotiated" protocol version to use in the svc state
struct instead of the connection struct.

The generic concept of a connection version is going away in favour of
bundle-class versions.

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