Bluetooth: Add support for mode 0x02 for mgmt_set_debug_keys

This patch adds a new valid mode 0x02 for the mgmt_set_debug_keys
command. The 0x02 mode sets the HCI_USE_DEBUG_KEYS flag which makes us
always use debug keys for pairing.

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