Bluetooth: Fix __hci_req_sync() handling of empty requests

If a request callback doesn't send any commands __hci_req_sync() should
fail imediately instead of waiting for the inevitable timeout to occur.
This is particularly important once we start creating requests with
conditional command sending which can potentially result in no commands
being sent at all.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>