rgbled: don't try the same bus twice

on PX4v1 the external I2C bus is the same as the LED bus

src/drivers/rgbled/rgbled.cpp

@@ -616,6 +616,9 @@ rgbled_main(int argc, char *argv[])
 
 			if (g_rgbled == nullptr) {
 				// fall back to default bus
+				if (PX4_I2C_BUS_LED == PX4_I2C_BUS_EXPANSION) {
+					errx(1, "init failed");
+				}
 				i2cdevice = PX4_I2C_BUS_LED;
 			}
 		}