Bluetooth: Controller: df: Do not send IQ rep for PDU without CTE

Check if the CTE was discovered by Radio peripheral should be
done with use of CTEPRESENT event. Samples count should not
be used for that purpose. If samples count is a value different
than zero for PDUs that don't have CTE, unwanted IQ samples
report will be generated. CTEPRESENT event is set only in case the
CTEInfo filed was correctly parsed by Radio peripheral during
PDU reception.

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