Do not access NV device if not present

Prevents NULL pointer dereference on pang10 when removing module.

system76_nv_hda.c

@@ -162,6 +162,9 @@ static int __init nv_hda_init(struct device *dev) {
 }
 
 static void __exit nv_hda_exit(void) {
+    if (dis_dev == NULL)
+        return;
+
     dis_dev_get();
 
     nv_hda_off();