Bluetooth: host: Allow to initiate pairing without OOB present

When OOB callbacks are present it is possible to achieve authenticated
pairing without having the remote OOB data present. Using OOB with
LE Secure Connection only one side of the pairing procedure is required
to have the OOB data present. If we have given the remote our OOB data
then pairing can proceed.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>