fix availableForWrite() signature

Author: hathach

cores/arduino/Adafruit_TinyUSB_Core/Adafruit_USBD_CDC.cpp

@@ -126,7 +126,7 @@ size_t Adafruit_USBD_CDC::write(const uint8_t *buffer, size_t size)
   return size - remain;
 }
 
-size_t Adafruit_USBD_CDC::availableForWrite(void)
+int Adafruit_USBD_CDC::availableForWrite(void)
 {
   return tud_cdc_write_available();
 }

cores/arduino/Adafruit_TinyUSB_Core/Adafruit_USBD_CDC.h

@@ -51,7 +51,7 @@ class Adafruit_USBD_CDC : public Stream, Adafruit_USBD_Interface
 	size_t write(const char *buffer, size_t size) {
 	  return write((const uint8_t *)buffer, size);
 	}
-	size_t availableForWrite(void);
+	virtual int availableForWrite(void);
 	operator bool();
 };