Bluetooth: Controller: Fix missing event counter sub by one

The event counter value for connection and periodic advertising
events is updated to next value during event preparation (in one
of prepare_cb functions in LLL).
IQ report is prepared after event preparation, so value of event
counter is ahead of the current event value by one, hence it has
to be subtracted.

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