Fix unnecessary memory usage in radio

lib/include/srslte/radio/radio.h

@@ -140,7 +140,7 @@ class radio {
 
   srslte_rf_t rf_device;
 
-  const static uint32_t burst_preamble_max_samples = 30720000;  // 30.72 MHz is maximum frequency
+  const static uint32_t burst_preamble_max_samples = 4096;
   double burst_preamble_sec;// Start of burst preamble time (off->on RF transition time)
   srslte_timestamp_t end_of_burst_time;
   bool is_start_of_burst;

lib/src/radio/radio.cc

@@ -359,7 +359,7 @@ void radio::set_tx_srate(double srate)
   burst_preamble_samples = (uint32_t) (cur_tx_srate * burst_preamble_sec);
   if (burst_preamble_samples > burst_preamble_max_samples) {
     burst_preamble_samples = burst_preamble_max_samples;
-    fprintf(stderr, "Error setting TX srate %.1f MHz. Maximum frequency for zero prepadding is 30.72 MHz\n", srate*1e-6);
+    fprintf(stderr, "Error setting TX srate %.1f MHz. Maximum burst preamble samples: %d, requested: %d\n", srate*1e-6, burst_preamble_max_samples, burst_preamble_samples  );
   }
   burst_preamble_time_rounded = (double) burst_preamble_samples/cur_tx_srate;