Make slow SPI speeds work on the auxiliary SPI master

The auxiliary SPI driver could get desynchronized with the hardware
when reading using low bit rates.  It was only looking to see if the
TX queue was empty, which doesn't actually tell you if the hardware is
done doing stuff or not.

lib/cpp/mjbots/pi3hat/pi3hat.cc

@@ -631,7 +631,13 @@ class AuxSpi {
       spi_->io = value;
     }
 
-    while ((spi_->stat & AUXSPI_STAT_TX_EMPTY) == 0);
+    while (true) {
+      const auto stat = spi_->stat;
+      if ((stat & AUXSPI_STAT_BUSY) == 0 &&
+          (stat & AUXSPI_STAT_TX_EMPTY) != 0) {
+        break;
+      }
+    }
 
     if (size == 0) { return; }