Bluetooth: LE scan should send Discovering events

Send MGMT Discovering events once LE scan starts/stops so the
userspace can track when local adapters are discovering LE devices.

This way, we also keep the same behavior of inquiry which sends MGMT
Discovering events once inquiry starts/stops even if it is triggered
by an external tool (e.g. hcitool).

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>