SITL: correct I2C multi-bus support

libraries/SITL/SIM_I2C.cpp

@@ -98,16 +98,18 @@ int I2C::ioctl_rdwr(i2c_rdwr_ioctl_data *data)
     for (uint8_t i=0; i<data->nmsgs; i++) {
         const i2c_msg &msg = data->msgs[i];
         const uint8_t addr = msg.addr;
+        const uint8_t bus = msg.bus;
         bool handled = false;
-        for (auto dev_at_address : i2c_devices) {
-            // where's the bus check?!
-            if (dev_at_address.addr == addr) {
-                ret = dev_at_address.device.rdwr(data);
+        for (auto &dev_at_address : i2c_devices) {
+            if (dev_at_address.addr == addr &&
+                dev_at_address.bus == bus) {
                 handled = true;
+                ret = dev_at_address.device.rdwr(data);
+                break;  // uniqueness of addr/bus device is ensured in init()
             }
         }
         if (!handled) {
-            ::fprintf(stderr, "Unhandled i2c message: bus=%u addr=0x%02x flags=%u len=%u\n", msg.bus, msg.addr, msg.flags, msg.len);
+//            ::fprintf(stderr, "Unhandled i2c message: bus=%u addr=0x%02x flags=%u len=%u\n", msg.bus, msg.addr, msg.flags, msg.len);
             return -1;  // ?!
         }
     }