srsenb_sib.conf: Fix warning about wrong prach_freq_offset value

WARNING: Invalid PRACH configuration - prach=(2, 8) does not fit into the eNB PRBs=(0, 6).
       Consider changing the "prach_freq_offset" value to 0 in the sib.conf file when using 6 PRBs.

Change-Id: Ie4374faca62a4d0289f78674eadec7eb5d045dfd

src/osmo_gsm_tester/templates/srsenb_sib.conf.tmpl

@@ -46,7 +46,11 @@ sib2 =
             {
                 high_speed_flag = false;
                 prach_config_index = 3;
+% if int(enb['num_prb']) <= 6:
+                prach_freq_offset = 0;
+% else:
                 prach_freq_offset = 2;
+% endif
                 zero_correlation_zone_config = 5;
             };
         };