nvme-auth: use transformed key size to create resp

This does not change current behaviour as the driver currently
verifies that the secret size is the same size as the length of
the transformation hash.

Co-developed-by: Akash Appaiah <Akash.Appaiah@dell.com>
Signed-off-by: Akash Appaiah <Akash.Appaiah@dell.com>
Signed-off-by: Mark O'Donovan <shiftee@posteo.net>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>