[USBHost] Add constructor and refactor privates

Author: manchoz

src/Arduino_MachineControl.h

@@ -527,18 +527,23 @@ extern RtcControllerClass rtc_controller;
  */
 class USBClass {
 public:
+	USBClass()
+		: _power{PB_14, 0}
+		, _usbflag{PB_15}
+		{};
+		
 	/**
 	 * Enable power to USBA VBUS. 
 	 */
 	void powerEnable() {
-		power = 0;
+		_power = 0;
 	}
 
 	/**
 	 * Disable power to USBA VBUS.  
 	 */
 	void powerDisable() {
-		power = 1;
+		_power = 1;
 	}
 
 	/**
@@ -547,12 +552,12 @@ class USBClass {
 	 * @return true if OK, false if fault
 	 */
 	bool vflagRead() {
-		return usbflag;
+		return _usbflag;
 	}
 
 private:
-	mbed::DigitalOut power = mbed::DigitalOut(PB_14);
-	mbed::DigitalIn usbflag = mbed::DigitalIn(PB_15);
+	mbed::DigitalOut _power;
+	mbed::DigitalIn _usbflag;
 };