Bluetooth: Simplify logic for checking for SMP completion

Now that smp->remote_key_dist is tracking the keys we're still waiting
for we can use it to simplify the logic for checking whether we're done
with key distribution or not. At the same time the reliance on the
"force" parameter of smp_distribute_keys goes away and it can completely
be removed in a subsequent patch.

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