Fix bug in templated attach function

The arguments passed to `Callback<void()>`'s constructor were the wrong way round,
thus preventing the `attach` function from being instantiated.
This patch corrects that by switching the order of the arguments.

Author: Pharap

features/unsupported/USBDevice/USBSerial/USBSerial.h

@@ -126,8 +126,8 @@ class USBSerial: public USBCDC, public Stream {
      */
     template<typename T>
     void attach(T* tptr, void (T::*mptr)(void)) {
-        if((mptr != NULL) && (tptr != NULL)) {
-            rx = Callback<void()>(mptr, tptr);
+        if((tptr != NULL) && (mptr != NULL)) {
+            rx = Callback<void()>(tptr, mptr);
         }
     }