AP_RangeFinder: fix LightwareI2C conversion to I2CDevice

libraries/AP_RangeFinder/AP_RangeFinder_LightWareI2C.cpp

@@ -62,8 +62,9 @@ bool AP_RangeFinder_LightWareI2C::get_reading(uint16_t &reading_cm)
         return false;
     }
 
+
     // exit immediately if we can't take the semaphore
-    if (!_dev || _dev->get_semaphore()->take(1)) {
+    if (!_dev || !_dev->get_semaphore()->take(1)) {
         return false;
     }