gianfar: Fix boot hangs while bringing up gianfar ethernet

Ira Snyder found that commit 8c7396aebb68994c0519e438eecdf4d5fa9c7844
"gianfar: Merge Tx and Rx interrupt for scheduling clean up ring" can
cause hangs. It's because there was removed clearing of interrupts in
gfar_schedule_cleanup() (which is called by an interrupt handler) in
case when netif scheduling has been disabled. This patch brings back
this action and a comment.

Reported-by: Ira Snyder <iws@ovro.caltech.edu>
Reported-by: Peter Korsgaard <jacmet@sunsite.dk>
Bisected-by: Ira Snyder <iws@ovro.caltech.edu>
Tested-by: Peter Korsgaard <jacmet@sunsite.dk>
Tested-by: Ira Snyder <iws@ovro.caltech.edu>
Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/gianfar.c

@@ -1629,6 +1629,12 @@ static void gfar_schedule_cleanup(struct net_device *dev)
 	if (netif_rx_schedule_prep(&priv->napi)) {
 		gfar_write(&priv->regs->imask, IMASK_RTX_DISABLED);
 		__netif_rx_schedule(&priv->napi);
+	} else {
+		/*
+		 * Clear IEVENT, so interrupts aren't called again
+		 * because of the packets that have already arrived.
+		 */
+		gfar_write(&priv->regs->ievent, IEVENT_RTX_MASK);
 	}
 
 	spin_unlock(&priv->rxlock);