This restores behaviour (including the comment) from now-removed
hci_request.c, and also matches existing code for active scanning.

Without this, the duplicates filter is always active when passive
scanning, which makes it impossible to work with devices that send
nontrivial dynamic data in their advertisement reports.

Fixes: abfeea47 ("Bluetooth: hci_sync: Convert MGMT_OP_START_DISCOVERY")
Signed-off-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>