fix prach packet in if4p5 UDP

targets/ARCH/ETHERNET/USERSPACE/LIB/eth_udp.c

@@ -208,7 +208,9 @@ int trx_eth_write_udp_IF4p5(openair0_device *device, openair0_timestamp timestam
   if (flags == IF4p5_PDLFFT) {
     packet_size = UDP_IF4p5_PDLFFT_SIZE_BYTES(nblocks);    
   } else if (flags == IF4p5_PULFFT) {
-    packet_size = UDP_IF4p5_PULFFT_SIZE_BYTES(nblocks);    
+    packet_size = UDP_IF4p5_PULFFT_SIZE_BYTES(nblocks); 
+  } else if (flags == IF4p5_PRACH) {  
+    packet_size = UDP_IF4p5_PRACH_SIZE_BYTES;   
   } else {
     printf("trx_eth_write_udp_IF4p5: unknown flags %d\n",flags);
     return(-1);