Bluetooth: hci_sync: Push sync command cancellation to workqueue

syzbot reported that hci_cmd_sync_cancel may sleep from the wrong
context. To avoid this, create a new work item that pushes the relevant
parts into a different context.

Note that we keep the old implementation with the name
__hci_cmd_sync_cancel as the sleeping behaviour is desired in some
cases.

Reported-and-tested-by:  <syzbot+485cc00ea7cf41dfdbf1@syzkaller.appspotmail.com>
Fixes: c97a747e ("Bluetooth: btusb: Cancel sync commands for certain URB errors")
Signed-off-by: Benjamin Berg <bberg@redhat.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>