Bluetooth: Move discovery macros to hci_core.h

Some of discovery macros will be used in hci_core so we need to
define them in common place such as hci_core.h. Thus, this patch
moves discovery macros to hci_core.h and also adds the DISCOV_
prefix to them.

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
Acked-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>