Bluetooth: Add hci_le_scan()

We are not supposed to block in start_discovery() because
start_discovery code is running in write() syscall context
and this would block the write operation on the mgmt socket.
This way, we cannot directly call hci_do_le_scan() to scan
LE devices in start_discovery(). To overcome this issue a
derefered work (hdev->le_scan) was created so we can properly
call hci_do_le_scan().

The helper function hci_le_scan() simply set LE scan parameters
and queue hdev->le_scan work. The work is queued on system_long_wq
since it can sleep for a few seconds in the worst case (timeout).

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>