greybus: connection: fix oops after failed init

Make sure not to call connection_exit for connections that have never
been initialised (e.g. due to failure to init).

This fixes oopses due to null-dereferences and use-after-free in
connection_exit callbacks (e.g. trying to remove a gpio-chip that has
never been added) when the bundle and interface are ultimately
destroyed.

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