Fix USBSerial for large receives

When GDB would send packets larger than 256 bytes, the rx_buffer in
USBSerial would fill before it completely emptied the underlying USBCDC
receive buffer in the onInterrupt() callback. Since the underlying
USBCDC receive buffer still had data in it, it would never call
onInterrupt() again and MRI would never receive any more data from GDB.

I just added a call to onInterrupt() to any USBSerial methods which
relate to receiving data to make sure that the underlying USBCDC
receive buffer has a chance to be emptied so that more data can be
received from the PC.

Author: adamgreen

cores/arduino/USB/PluggableUSBSerial.h

@@ -221,14 +221,17 @@ class USBSerial: public USBCDC, public ::mbed::Stream, public HardwareSerial {
     }
 
     int available(void) {
+        onInterrupt();
         return rx_buffer.available();
     }
 
     int peek(void) {
+        onInterrupt();
         return rx_buffer.peek();
     }
 
     int read(void) {
+        onInterrupt();
         return rx_buffer.read_char();
     }