AP_HAL_Linux: Use from() method for downcast in Util class

libraries/AP_HAL_Linux/GPIO_RPI.cpp

@@ -24,7 +24,7 @@ LinuxGPIO_RPI::LinuxGPIO_RPI()
 
 void LinuxGPIO_RPI::init()
 {
-    int rpi_version = static_cast<LinuxUtilNavio*>(hal.util)->get_rpi_version();
+    int rpi_version = LinuxUtilNavio::from(hal.util)->get_rpi_version();
     uint32_t gpio_address = rpi_version == 1 ? GPIO_BASE(BCM2708_PERI_BASE)   : GPIO_BASE(BCM2709_PERI_BASE);
     uint32_t pwm_address  = rpi_version == 1 ? PWM_BASE(BCM2708_PERI_BASE)    : PWM_BASE(BCM2709_PERI_BASE);
     uint32_t clk_address  = rpi_version == 1 ? CLOCK_BASE(BCM2708_PERI_BASE)  : CLOCK_BASE(BCM2709_PERI_BASE);

libraries/AP_HAL_Linux/RCInput_Navio.cpp

@@ -392,7 +392,7 @@ LinuxRCInput_Navio::LinuxRCInput_Navio():
     last_signal(228),
     state(RCIN_NAVIO_INITIAL_STATE)
 {
-    int version = static_cast<LinuxUtilNavio*>(hal.util)->get_rpi_version();
+    int version = LinuxUtilNavio::from(hal.util)->get_rpi_version();
     set_physical_addresses(version);
 
     //Init memory for buffer and for DMA control blocks. See comments in "init_ctrl_data()" to understand values "2" and "113"

libraries/AP_HAL_Linux/Util_Navio.h

@@ -5,6 +5,11 @@
 class Linux::LinuxUtilNavio : public Linux::LinuxUtil {
 public:
     LinuxUtilNavio();
+
+    static LinuxUtilNavio *from(AP_HAL::Util *util) {
+        return static_cast<LinuxUtilNavio*>(util);
+    }
+
     /* return the Raspberry Pi version */
     int get_rpi_version() const;