fbd: Fix a bug where vt_fb_attach() success would be considered a failure

vt_fb_attach() currently always returns 0, but it could return a code
defined in errno.h. However, it doesn't return a CN_* code. So checking
its return value against CN_DEAD (which is 0) is incorrect, and in this
case, a success becomes a failure.

The consequence was unimportant, because the caller (drm_fb_helper.c)
would only log an error message in this case. The console would still
work.

Approved by:	nwhitehorn

sys/dev/fb/fbd.c

@@ -246,8 +246,9 @@ fbd_register(struct fb_info* info)
 		return (err);
 
 	if (first) {
-		if (vt_fb_attach(info) == CN_DEAD)
-			return (ENXIO);
+		err = vt_fb_attach(info);
+		if (err)
+			return (err);
 	}
 
 	return (0);