Bluetooth: Track not yet received keys in SMP

To make is easier to track which keys we've received and which ones
we're still waiting for simply clear the corresponding key bits from
smp->remote_key_dist as they get received. This will allow us to
simplify the code for checking for SMP completion in subsequent patches.

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