Bluetooth: controller: split: Fix DLE preparation routine

This fixes the EBQ tests 129 and 130. These tests check behaviour for
the DLE procedure when Encoded PHY or 2M PHY are not supported.
See also BT core spec. Version 5.1, Vol6, Part B, Section 5.1.9

Signed-off-by: Andries Kruithof <Andries.Kruithof@nordicsemi.no>