Bluetooth: Add macros for filter duplicates values

This patch adds macros for filter_duplicates parameter values from
HCI LE Set Scan Enable command. It also fixes le_scan_enable_req
function so it uses the LE_SCAN_FILTER_DUP_ENABLE macro instead of
a magic number.

The LE_SCAN_FILTER_DUP_DISABLE was also defined since it will be
required to properly support the GAP Observer Role.

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>