USB-CDC: Increased buffer size to 256 bytes

This change improves read performance when massive data is sent
via USB by exploiting the hardware capability to handle multi-packet
transfers autonomously.

cores/arduino/USB/USBCore.cpp

@@ -427,7 +427,7 @@ void USBDeviceClass::initEP(uint32_t ep, uint32_t config)
 	}
 	else if (config == (USB_ENDPOINT_TYPE_BULK | USB_ENDPOINT_OUT(0)))
 	{
-		epHandlers[ep] = new DoubleBufferedEPOutHandler(usbd, ep, 64);
+		epHandlers[ep] = new DoubleBufferedEPOutHandler(usbd, ep, 256);
 	}
 	else if (config == (USB_ENDPOINT_TYPE_BULK | USB_ENDPOINT_IN(0)))
 	{