Bluetooth: Use debug keys for SMP when HCI_USE_DEBUG_KEYS is set

The HCI_USE_DEBUG_KEYS flag is intended to force our side to always use
debug keys for pairing. This means both BR/EDR SSP as well as SMP with
LE Secure Connections. This patch updates the SMP code to use the debug
keys instead of generating a random local key pair when the flag is set.

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