greybus: connection: destroy connection on failing to bind it

gb_connection_bind_protocol() returns proper error codes now and we
should destroy the connection on failures.

This change also fixes a NULL deref on hotplug when the control connection fails
to initialize.

Signed-off-by: Fabien Parent <fparent@baylibre.com>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>