Don't initialize COM2.

At least not until we implement MCHK handling.  The current QEMU will
(properly) MCHK when accessing a non-existant device.
diff --git a/uart.c b/uart.c
index cf2615e..61e467a 100644
--- a/uart.c
+++ b/uart.c
@@ -139,5 +139,5 @@
 void uart_init(void)
 {
 	uart_init_line(COM1, SERIAL_SPEED);
-	uart_init_line(COM2, SERIAL_SPEED);
+	/* uart_init_line(COM2, SERIAL_SPEED); */
 }