greybus: connection: bind protocol at init

Bind protocol at connection init.

This is an intermediate step in separating connection creation and
enabling.

Note that the protocol is currently still unbound when the connection is
destroyed.

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