When initializing auth context, there may be no secrets passed
by the user. Make return code explicit when returning successfully.

smatch warnings:
drivers/nvme/host/auth.c:950 nvme_auth_init_ctrl() warn: missing error code? 'ret'
Reported-by: Nkernel test robot <lkp@intel.com>
Reported-by: NDan Carpenter <error27@gmail.com>
Signed-off-by: NSagi Grimberg <sagi@grimberg.me>
Signed-off-by: NChristoph Hellwig <hch@lst.de>