Fix up all the newly imported Teensy code

Change-Id: Id051083a16336617add1d5f16fc7b7d78a30a5fe
diff --git a/motors/usb/usb_mem.h b/motors/usb/usb_mem.h
index 8064488..b6014b5 100644
--- a/motors/usb/usb_mem.h
+++ b/motors/usb/usb_mem.h
@@ -45,7 +45,7 @@
 #endif
 
 usb_packet_t * usb_malloc(void);
-void usb_free(usb_packet_t *p);
+void usb_free(const usb_packet_t *p);
 
 #ifdef __cplusplus
 }