Fix compiler warnings with USB audio

teensy3/usb_audio.cpp

 	while ((src < target - 2)) {
 		uint32_t n1 = *src++;
 		uint32_t n = *src++;
-		*(uint32_t *)left = n1 & 0xFFFF | ((n & 0xFFFF) << 16);
+		*(uint32_t *)left = (n1 & 0xFFFF) | ((n & 0xFFFF) << 16);
 		left+=2;
 		*(uint32_t *)right = (n1 >> 16) | ((n & 0xFFFF0000)) ;
 		right+=2;

teensy3/usb_dev.c

 #include "kinetis.h"
 //#include "HardwareSerial.h"
 #include "usb_mem.h"
+#include <string.h> // for memset
 
 // buffer descriptor table
 

teensy3/usb_dev.h

 extern uint8_t usb_audio_transmit_setting;
 extern void usb_audio_receive_callback(unsigned int len);
 extern unsigned int usb_audio_transmit_callback(void);
+extern int usb_audio_get_feature(void *stp, uint8_t *data, uint32_t *datalen);
+extern int usb_audio_set_feature(void *stp, uint8_t *buf);
 #endif
 
 #ifdef MULTITOUCH_INTERFACE