Bluetooth: Use OOB key pair for LE SC pairing with OOB method

The OOB public and secret key pair is different from the non-OOB pairing
procedure. SO when OOB method is in use, then use this key pair instead
of generating a new one.

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