Bluetooth: Controller: Fix redundant node rx reservation

Fix redundant node rx reservation when receiving Periodic
Advertising Sync chain PDUs. Only the reception of first
pdu in chain, AUX_SYNC_IND PDU, needs extra node rx
reservation to be allocated for generation of incomplete
data status in case of incomplete reception of chain PDUs.

Co-authored-by: Piotr Pryga <piotr.pryga@nordicsemi.no>
Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>