greybus: connection: set request handlers at creation

Set the connection request handler at creation rather than when enabling
the connection.

This is possible now that connections are created by the drivers that
use them rather than by core at manifest parsing 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>