xtensa/platforms/iss/simdisk: 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.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Acked-by: Max Filippov <jcmvbkbc@gmail.com>
Link: https://lore.kernel.org/r/20210927220110.1066271-7-mcgrof@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>