abcio: allow an I/O device to be unregistered

Allow unregistering an abcbus I/O device by calling abc_register()
with a NULL pointer.

rv32/abcio.c

@@ -194,7 +194,7 @@ void abc_register(struct abc_dev *dev, unsigned int devsel)
 
     irqmask_t irqmask = mask_irq(ABC_IRQ);
 
-    if (!dev->inp_cnt)
+    if (dev && !dev->inp_cnt)
 	dev->inp_data[0] = dev->inp_data_def;
 
     abc_device[devsel] = dev;