Correct PNF config: sl_ahead to 2

Using nFAPI introduces additional delays. At the same time, RFsim does
not run in real-time, and we can allow a low sl_ahead, as RFsim will
"wait" for the gNB.

Therefore, reduce sl_ahead to ensure that the response window does not
run out during handling of RA in nFAPI.

targets/PROJECTS/GENERIC-NR-5GC/CONF/gnb-pnf.band78.rfsim.conf

@@ -22,6 +22,7 @@ RUs = ({
   att_tx     = 0
   att_rx     = 0;
   bands      = [78];
+  sl_ahead = 2;
   max_pdschReferenceSignalPower = -27;
   max_rxgain = 125;
 });