net: systemport: fix TX NAPI work done return value

Although we do not limit the number of packets the TX completion
function bcm_sysport_tx_reclaim() is allowed to reclaim, we were still
using its return value as-is. This means that we could hit the WARN() in
net/core/dev.c where work_done >= budget.

Make sure we do exit the NAPI context when the TX ring is empty, and
pretend there was no work to do.
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/broadcom/bcmsysport.c

@@ -654,13 +654,13 @@ static int bcm_sysport_tx_poll(struct napi_struct *napi, int budget)
 
 	work_done = bcm_sysport_tx_reclaim(ring->priv, ring);
 
-	if (work_done < budget) {
+	if (work_done == 0) {
 		napi_complete(napi);
 		/* re-enable TX interrupt */
 		intrl2_1_mask_clear(ring->priv, BIT(ring->index));
 	}
 
-	return work_done;
+	return 0;
 }
 
 static void bcm_sysport_tx_reclaim_all(struct bcm_sysport_priv *priv)