s390/qeth: clean up initial MTU determination

1. Drop the support for Token Ring,
2. use the ETH_DATA_LEN macro for the default L2 MTU,
3. handle OSM via the default case (as OSM is L2-only), and
4. document why the L3 MTU is reduced.
Signed-off-by: Julian Wiedmann <jwi@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/s390/net/qeth_core_main.c

@@ -2219,18 +2219,12 @@ static int qeth_get_initial_mtu_for_card(struct qeth_card *card)
 	case QETH_CARD_TYPE_IQD:
 		return card->info.max_mtu;
 	case QETH_CARD_TYPE_OSD:
-		switch (card->info.link_type) {
-		case QETH_LINK_TYPE_HSTR:
-		case QETH_LINK_TYPE_LANE_TR:
-			return 2000;
-		default:
-			return card->options.layer2 ? 1500 : 1492;
-		}
-	case QETH_CARD_TYPE_OSM:
 	case QETH_CARD_TYPE_OSX:
-		return card->options.layer2 ? 1500 : 1492;
+		if (!card->options.layer2)
+			return ETH_DATA_LEN - 8; /* L3: allow for LLC + SNAP */
+		/* fall through */
 	default:
-		return 1500;
+		return ETH_DATA_LEN;
 	}
 }