Bluetooth: Controller: df: Fix wrong value of event counter in iq report

HCI_LE_Connectionless_IQ_Report and HCI_LE_Connection_IQ_Report
events has fields that store event counter values: Periodic_Event_-
Counter and Event_Counter, respectively.
Values assigned to this HCI events are taken directly from
lower link layer objects. In case of too slow processing of
HCI events values provided by LLL contexts could be changed before
HCI sends events to Host. That can happen e.g. in case of short
periodic advertising interval and slow UART baudrate.
In such situation event counter values will be corrupted.

To prevent that situation, event counter values connected with
IQ sample reports, has to be stored in node_rx_iq_report when
a report instance is filled in LLL.

This commit introduces required changes.

Signed-off-by: Piotr Pryga <piotr.pryga@nordicsemi.no>