Bluetooth: Controller: df: CP bit not cleared in DF conn mode RX

In direction finding connected mode there is a CP bit that is set
data PDU header. The bit was initialized only if CTE transmission
was enabled. In case of reception of a CTE the bit was available
in PDUs but not initialized.
That caused issues in connection maintenance if PDU memory buffers
were reused. PDU were malformed and connections were lost.

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