block/sx8: add error handling support for add_disk()

We never checked for errors on add_disk() as this function
returned void. Now that this is fixed, use the shiny new
error handling.

A completion is used to notify the initial probe what is
happening and so we must defer error handling on completion.
Do this by remembering the error and using the shared cleanup
function.

The tags are shared and so are hanlded later for the
driver already.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>