Bluetooth: Use struct delayed_work for HCI command timeout

Since the whole HCI command, event and data packet processing has been
migrated to use workqueues instead of tasklets, it makes sense to use
struct delayed_work instead of struct timer_list for the timeout
handling. This patch converts the hdev->cmd_timer to use workqueue
as well.

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