greybus: loopback: add more clean up when init connection fails

It should remove the object from sysfs when loopback
connection init error.

Signed-off-by: Phong Tran <tranmanphong@gmail.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>