HAL_VRBRAIN: implement _timer_tick in UARTDriver

libraries/AP_HAL_VRBRAIN/Scheduler.cpp

@@ -363,12 +363,12 @@ void *VRBRAINScheduler::_uart_thread(void *arg)
         sched->delay_microseconds_semaphore(1000);
 
         // process any pending serial bytes
-        ((VRBRAINUARTDriver *)hal.uartA)->_timer_tick();
-        ((VRBRAINUARTDriver *)hal.uartB)->_timer_tick();
-        ((VRBRAINUARTDriver *)hal.uartC)->_timer_tick();
-        ((VRBRAINUARTDriver *)hal.uartD)->_timer_tick();
-        ((VRBRAINUARTDriver *)hal.uartE)->_timer_tick();
-        ((VRBRAINUARTDriver *)hal.uartF)->_timer_tick();
+        hal.uartA->_timer_tick();
+        hal.uartB->_timer_tick();
+        hal.uartC->_timer_tick();
+        hal.uartD->_timer_tick();
+        hal.uartE->_timer_tick();
+        hal.uartF->_timer_tick();
     }
     return nullptr;
 }