USRP: usrp-tx-write-thread: return number of samples written

At least writerProcessWaitingQueue() checks for the exact number of
samples written. In many other places, we check that the return is
non-zero.

The usrp-tx-write-thread functionality returned 0, which triggers
various problems. Return the number of samples, as in the "direct write"
case (if-block before else-block).

Co-authored-by: Sagar Arora <sagar.arora@openairinterface.org>

radio/USRP/usrp_lib.cpp

@@ -553,7 +553,7 @@ static int trx_usrp_write(openair0_device_t *device,
       LOG_D(HW, "Signaling TX TS %llu\n", (unsigned long long)timestamp);
       pthread_cond_signal(&write_thread->cond_write);
       pthread_mutex_unlock(&write_thread->mutex_write);
-      return 0;
+      return nsamps;
     }
 }