HAL_PX4: fixed DSM bind on Pixracer

libraries/AP_HAL_PX4/RCInput.cpp

@@ -121,7 +121,10 @@ bool PX4RCInput::rc_bind(int dsmMode)
 {
     int fd = open("/dev/px4io", 0);
     if (fd == -1) {
-        hal.console->printf("RCInput: failed to open /dev/px4io\n");
+        fd = open("/dev/px4fmu", 0);
+    }
+    if (fd == -1) {
+        hal.console->printf("RCInput: failed to open /dev/px4io or /dev/px4fmu\n");
         return false;
     }