greybus: legacy: look up protocol at connection creation

Look up legacy protocol at connection create instead of at init.

Note that we can now look up the protocol before even creating the
actual connection. This is needed to be able to set a connection request
handler when creating the connection rather than when enabling it.

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>