Bluetooth: Add LE SC support for responding to Pairing Confirm PDU

When LE SC is being used we should always respond to it by sending our
local random number. This patch adds a convenience function for it which
also contains a check for the pre-requisite public key exchange
completion

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