From: Mike Sinkovsky <redacted>
Date: Mon, 26 Mar 2012 15:07:56 +0600

+/*
+ * Frame size is hardwired to 1514 bytes (including header),
+ *  => MTU for 802.1Q frames must be set to 1500-4 = 1496
+ */
+#define W5300_HARD_MTU		1500

I've told you that you must set the VLAN challenged feature flag
(NETIF_F_VLAN_CHALLENGED) for devices which have this restriction.

Just set that, and remove this comment, since the feature bit setting
documents this restriction fully and perfectly.