Lower RFsim EAGAIN error message to debug message

We set the RFsim TCP socket to non-blocking. Thus, it may happen that we
cannot write() to the socket when the kernel cannot handle the request,
in which case we wait some time before trying again. This is the
correct/expected behavior.

Since this is *normal*, an error message is not justified; we did not
even print a message in this case before commit 21790f9c4f introduced it.

radio/rfsimulator/simulator.c

@@ -314,7 +314,7 @@ static void fullwrite(int fd, void *_buf, ssize_t count, rfsimulator_state_t *t)
         continue;
 
       if (errno == EAGAIN) {
-        LOG_E(HW, "write() failed, errno(%d)\n", errno);
+        LOG_D(HW, "write() failed, errno(%d)\n", errno);
         usleep(250);
         continue;
       } else