Bluetooth: Controller: Fix delayed ULL scheduling aux PDU reception

Fix delayed ULL scheduling an auxiliary PDU reception by
skipping it. When many bufferred ADV_EXT_IND are to be
processed, the ULL scheduling could get late and then cause
the scan aux LLL to detect increased overhead start timing.

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