Set correct master clock rate for N320

targets/ARCH/USRP/USERSPACE/LIB/usrp_lib.cpp

@@ -1030,8 +1030,12 @@ extern "C" {
     }
 
     if (device_adds[0].get("type") == "n3xx") {
-      printf("Found USRP n300\n");
+      const std::string product = device_adds[0].get("product");
+      printf("Found USRP %s\n", product.c_str());
       device->type=USRP_N300_DEV;
+      if (product == "n320")
+        usrp_master_clock = 245.76e6; // N320 does not support 122.88e6 master clock rate
+      else
         usrp_master_clock = 122.88e6;
       args += boost::str(boost::format(",master_clock_rate=%f") % usrp_master_clock);