Bluetooth: Fix SSP acceptor just-works confirmation without MITM

From the Bluetooth Core Specification 4.1 page 1958:

"if both devices have set the Authentication_Requirements parameter to
one of the MITM Protection Not Required options, authentication stage 1
shall function as if both devices set their IO capabilities to
DisplayOnly (e.g., Numeric comparison with automatic confirmation on
both devices)"

So far our implementation has done user confirmation for all just-works
cases regardless of the MITM requirements, however following the
specification to the word means that we should not be doing confirmation
when neither side has the MITM flag set.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Tested-by: Szymon Janc <szymon.janc@tieto.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Cc: stable@vger.kernel.org