move some platform-specific code out of the main wiiuse source

Initializing and cleaning up platform-specific fields in the wiimote
structure is now done in a separate routine.

src/io.h

@@ -49,6 +49,9 @@ extern "C" {
 /** @{ */
 void wiiuse_handshake(struct wiimote_t* wm, byte* data, uint16_t len);
 
+void wiiuse_init_platform_fields(struct wiimote_t* wm);
+void wiiuse_cleanup_platform_fields(struct wiimote_t* wm);
+
 int wiiuse_io_read(struct wiimote_t* wm);
 int wiiuse_io_write(struct wiimote_t* wm, byte* buf, int len);
 /** @} */