USBSerial: fix available() return size

Author: facchinm

cores/arduino/USB/PluggableUSBSerial.h

@@ -115,7 +115,7 @@ class USBSerial: public USBCDC, public ::mbed::Stream, public HardwareSerial {
     *
     * @returns the number of bytes available
     */
-    uint8_t _available();
+    uint32_t _available();
 
     /**
     * Check if the terminal is connected.

cores/arduino/USB/USBSerial.cpp

@@ -104,13 +104,13 @@ void USBSerial::data_rx()
     }
 }
 
-uint8_t USBSerial::_available()
+uint32_t USBSerial::_available()
 {
     USBCDC::lock();
 
-    uint8_t size = 0;
+    uint32_t size = 0;
     if (!_rx_in_progress) {
-        size = _rx_size > 0xFF ? 0xFF : _rx_size;
+        size = _rx_size > CDC_MAX_PACKET_SIZE ? CDC_MAX_PACKET_SIZE : _rx_size;
     }
 
     USBCDC::unlock();