Linux: changed CDev to VDev for virtual device implementation

To avoid confusion when a real device and a virtual device is
being used, changed CDev to VDev for Linux.

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

src/drivers/device/spi.h

@@ -50,7 +50,11 @@ namespace device __EXPORT
 /**
  * Abstract class for character device on SPI
  */
+#ifdef __PX4_NUTTX
 class __EXPORT SPI : public CDev
+#else
+class __EXPORT SPI : public VDev
+#endif
 {
 protected:
 	/**