nvme-auth: fix an error code in nvme_auth_process_dhchap_challenge()

This function was transitioned from returning NVMe status codes to
returning traditional kernel error codes.  However, this particular
return now accidentally returns positive error codes like ENOMEM instead
of negative -ENOMEM.

Fixes: b0ef1b11 ("nvme-auth: don't use NVMe status codes")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>