Bluetooth: controller: split: Fix LENGTH_REQ PDU prepare

There is an obscure bug in the case that CFG_BT_CTLR_PHY is not defined;
when a feature-exchange already has happened the lr->max_tx_time and
lr->max_rx_time are not calculated.
This bug is fixed by this commit.

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