Bluetooth: btusb: Add internal recv_event callback for event processing

Some new upcoming drivers need to process HCI events or take extra
actions based on them before handing the event to the Bluetooth core
for processing. The new recv_event callback allows exactly such an
internal behavior.

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