floppy: properly handle failure on add_disk loop

On floppy initialization, if something failed inside the loop we call
add_disk, there was no cleanup of previous iterations in the error
handling.

Cc: stable@vger.kernel.org
Signed-off-by: Herton Ronaldo Krzesinski <herton.krzesinski@canonical.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>