block: sed-opal: don't repeat opal_discovery0 in each steps array

Originally each of the opal functions that call next include
opal_discovery0 in the array of steps. This is superfluous and
can be done always inside next.

Acked-by: Jon Derrick <jonathan.derrick@intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Scott Bauer <sbauer@plzdonthack.me>
Signed-off-by: David Kozub <zub@linux.fjfi.cvut.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>