Bluetooth: Fix SMP user passkey notification mgmt event

When performing SMP pairing with MITM protection one side needs to
enter the passkey while the other side displays to the user what needs
to be entered. Nowhere in the SMP specification does it say that the
displaying side needs to any kind of confirmation of the passkey, even
though a code comment in smp.c implies this.

This patch removes the misleading comment and converts the code to use
the passkey notification mgmt event instead of the passkey confirmation
mgmt event.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>