Bluetooth: Track received events in hdev

This patch adds tracking of received HCI events to the hci_dev struct.
This is necessary so that a subsequent patch can implement a function
for sending a single command synchronously and returning the resulting
command complete parameters in the function return value.

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