Bluetooth: Controller: Do not clear timer clear subscription until done

Do not clear timer clear subscription until the radio event
is done. This is required for single timer switching when
periodic chains are to be received by LLL scheduling. Each
PDU in the train requires the timer to be cleared when the
parent PDU has been received such that reception of
auxiliary PDU's radio_tmr_start_us() can use the relative
offset since last PDU reception.

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