USB-CDC: Avoid need to send ZLP by sending up to (EPX_SIZE-1) bytes at a time

cores/arduino/USB/USBCore.cpp

@@ -627,8 +627,8 @@ uint32_t USBDeviceClass::send(uint32_t ep, const void *data, uint32_t len)
 	// Flash area
 	while (len != 0)
 	{
-		if (len >= 64) {
-			length = 64;
+		if (len >= EPX_SIZE) {
+			length = EPX_SIZE - 1;
 		} else {
 			length = len;
 		}