greybus: legacy: add initialized flag

Add initialized flag and use instead of the connection protocol pointer
to determine when the legacy connection has been initialised.

This is a step in moving legacy connection binding to
connection-creation time.

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>