Bluetooth: Convert local OOB data reading to use HCI request

Now that there's a HCI request API available where the callback receives
the resulting skb, we can convert the local OOB data reading to use this
new API. This patch does the necessary update in mgmt.c (which also
requires moving the callback higher up since it's now a static function)
and removes the custom calls from hci_event.c that are no-longer
necessary.

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