update tx_sample_advance for 10 MHz and USRP X300

Tests with our USRP X300, connected via PCIe, show that tx_sample_advance of 45 is slightly too much.
Unfortunately, transmitting the PRACH preamble too early is very bad, slightly too late is no problem.
So change this value from 45 to 40. This helps receiving the PRACH preamble reliably.

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

@@ -1125,7 +1125,7 @@ extern "C" {
   
       case 15360000:
         //openair0_cfg[0].samples_per_packet    = 2048;
-        openair0_cfg[0].tx_sample_advance     = 45;
+        openair0_cfg[0].tx_sample_advance     = 40;
         openair0_cfg[0].tx_bw                 = 10e6;
         openair0_cfg[0].rx_bw                 = 10e6;
         break;