Fix off-by-1 error in new Serial#.room()

teensy3/serial1.c

 
 	head = tx_buffer_head;
 	tail = tx_buffer_tail;
-	if (head >= tail) return TX_BUFFER_SIZE - head + tail;
-	return tail - head;
+	if (head >= tail) return TX_BUFFER_SIZE - 1 - head + tail;
+	return tail - head - 1;
 }
 
 int serial_available(void)

teensy3/serial2.c

 
 	head = tx_buffer_head;
 	tail = tx_buffer_tail;
-	if (head >= tail) return TX_BUFFER_SIZE - head + tail;
-	return tail - head;
+	if (head >= tail) return TX_BUFFER_SIZE - 1 - head + tail;
+	return tail - head - 1;
 }
 
 int serial2_available(void)

teensy3/serial3.c

 
 	head = tx_buffer_head;
 	tail = tx_buffer_tail;
-	if (head >= tail) return TX_BUFFER_SIZE - head + tail;
-	return tail - head;
+	if (head >= tail) return TX_BUFFER_SIZE - 1 - head + tail;
+	return tail - head - 1;
 }
 
 int serial3_available(void)