Bluetooth: Controller: Fix receiving spurious PDU in active scanning

Fix receiving spurious overlapping ADV_IND and SCAN_IND PDU
when LLL state expects to receive SCAN_RSP PDU. Explicitly
check that these PDUs are not processed when in active
scanning and waiting for SCAN_RSP PDUs.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>