next-cube.c: add dummy Ethernet register to allow diagnostic to timeout

Add a dummy register at address 0x6000 in the MMIO memory region to allow the
initial diagnostic test to timeout rather than getting stuck in a loop
continuously writing "en_write: tx not ready" to the console.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Tested-by: Thomas Huth <huth@tuxfamily.org>
Message-ID: <20231220131641.592826-2-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Thomas Huth <huth@tuxfamily.org>

hw/m68k/next-cube.c

@@ -429,6 +429,10 @@ static uint32_t scr_readb(NeXTPC *s, hwaddr addr)
         /* Hack: We need to have this change consistently to make it work */
         return 0xFF & clock();
 
+    /* For now return dummy byte to allow the Ethernet test to timeout */
+    case 0x6000:
+        return 0xff;
+
     default:
         DPRINTF("BMAP Read B @ %x\n", (unsigned int)addr);
         return 0;