nvme-auth: update bi_directional flag

While setting chap->s2 to zero as part of secure channel
concatenation, the host missed out to disable the bi_directional
flag to indicate that controller authentication is not requested.
Fix the same.

Fixes: e88a7595 ("nvme-tcp: request secure channel concatenation")
Signed-off-by: Martin George <marting@netapp.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>