Bluetooth: Controller: Fix assert on aux LLL scheduled chain reception

Fix asserted in ULL due to incorrect resumption of scan
window when auxiliary channel chain PDU is LLL scheduled by
a ULL scheduled auxiliary channel PDU reception.

The issue is solved by having `is_chain_sched` flag in the
auxiliary channel scan context and using the already present
`is_aux_sched` in the primary channel scan context to
differentiate if the auxiliary PDU Rx ISR is to return back
to primary channel scan window or to close the auxiliary
chain PDU reception radio event.

Relates to #38146.

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