AP_HAL_Linux: AnalogIn_IIO: return fixed board voltage

Return board voltage as 5V so we don't fail prearm checks.

libraries/AP_HAL_Linux/AnalogIn_IIO.h

@@ -53,6 +53,5 @@ public:
     AP_HAL::AnalogSource* channel(int16_t n);
 
     // we don't yet know how to get the board voltage
-    float board_voltage(void) { return 0.0f; }
-
+    float board_voltage(void) { return 5.0f; }
 };