amiflop: 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. The caller for fd_alloc_disk() deals with
the rest of the cleanup like the tag.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Link: https://lore.kernel.org/r/20210927220302.1073499-7-mcgrof@kernel.org


Signed-off-by: Jens Axboe <axboe@kernel.dk>