net: xgene: Check negative return value of xgene_enet_get_ring_size()

xgene_enet_get_ring_size() returns a negative value in case of an error,
but its only caller in xgene_enet_create_desc_ring() currently uses the
return value directly as u32. Instead, check for a negative value first and
error out in case. Also move the call to xgene_enet_get_ring_size() before
devm_kzalloc() so we don't need to free anything in the error path.

This fixes the following issue reported by the Coverity Scanner:

** CID 1231336:  Improper use of negative value  (NEGATIVE_RETURNS)
/drivers/net/ethernet/apm/xgene/xgene_enet_main.c: 596 in xgene_enet_create_desc_ring()
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/apm/xgene/xgene_enet_main.c

@@ -581,7 +581,11 @@ static struct xgene_enet_desc_ring *xgene_enet_create_desc_ring(
 	struct xgene_enet_desc_ring *ring;
 	struct xgene_enet_pdata *pdata = netdev_priv(ndev);
 	struct device *dev = ndev_to_dev(ndev);
-	u32 size;
+	int size;
+
+	size = xgene_enet_get_ring_size(dev, cfgsize);
+	if (size < 0)
+		return NULL;
 
 	ring = devm_kzalloc(dev, sizeof(struct xgene_enet_desc_ring),
 			    GFP_KERNEL);
@@ -593,7 +597,6 @@ static struct xgene_enet_desc_ring *xgene_enet_create_desc_ring(
 	ring->cfgsize = cfgsize;
 	ring->id = ring_id;
 
-	size = xgene_enet_get_ring_size(dev, cfgsize);
 	ring->desc_addr = dma_zalloc_coherent(dev, size, &ring->dma,
 					      GFP_KERNEL);
 	if (!ring->desc_addr) {