update tx_sample_advance for 40 MHz and USRP X300 (3/4 sampling)

Testing RA with OAI gNB and UE both using X300 via PCIe link indicated a value of 15 was way too small.
The correct value for this setup seems to be 65.

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

@@ -1110,7 +1110,7 @@ extern "C" {
   
       case 46080000:
         //openair0_cfg[0].samples_per_packet    = 2048;
-        openair0_cfg[0].tx_sample_advance     = 15;
+        openair0_cfg[0].tx_sample_advance     = 65;
         openair0_cfg[0].tx_bw                 = 40e6;
         openair0_cfg[0].rx_bw                 = 40e6;
         break;