gianfar: Revive VLAN support

commit 77ecaf2d ("gianfar: Fix VLAN
HW feature related frame/buffer size calculation") wrongly removed
priv->vlgrp assignment, and now priv->vlgrp is always NULL.

This patch fixes the issue, plus fixes following sparse warning
introduced by the same commit:
gianfar.c:1406:13: warning: context imbalance in 'gfar_vlan_rx_register' - wrong count at exit

gfar_vlan_rx_register() checks for "if (old_grp == grp)" and tries
to return w/o dropping the lock.

According to net/8021q/vlan.c VLAN core issues rx_register() callback:
1. In register_vlan_dev() only on a newly created group;
2. In unregister_vlan_dev() only if the group becomes empty.

Thus the check in the gianfar driver isn't needed.
Signed-off-by: NAnton Vorontsov <avorontsov@ru.mvista.com>
Acked-by: NAndy Fleming <afleming@freescale.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/gianfar.c

@@ -1423,15 +1423,11 @@ static void gfar_vlan_rx_register(struct net_device *dev,
 {
 	struct gfar_private *priv = netdev_priv(dev);
 	unsigned long flags;
-	struct vlan_group *old_grp;
 	u32 tempval;
 
 	spin_lock_irqsave(&priv->rxlock, flags);
 
-	old_grp = priv->vlgrp;
-
-	if (old_grp == grp)
-		return;
+	priv->vlgrp = grp;
 
 	if (grp) {
 		/* Enable VLAN tag insertion */