Bluetooth: Add hci_cancel_le_scan() to hci_core

This patch adds to hci_core the hci_cancel_le_scan function which
should be used to cancel an ongoing LE scan.

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