ixgbe: Fix ixgbe_tx_map error path

Commit e5a43549f7a58509a91b299a51337d386697b92c (ixgbe: remove
skb_dma_map/unmap calls from driver) looks to have introduced a bug in
ixgbe_tx_map. If we get an error from a PCI DMA call, we loop backwards
through count until it becomes -1 and return that.

The caller of ixgbe_tx_map expects 0 on error, so return that instead.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/ixgbe/ixgbe_main.c b/drivers/net/ixgbe/ixgbe_main.c
index b5f64ad..37e2af0 100644
--- a/drivers/net/ixgbe/ixgbe_main.c
+++ b/drivers/net/ixgbe/ixgbe_main.c
@@ -5179,7 +5179,7 @@
 		ixgbe_unmap_and_free_tx_resource(adapter, tx_buffer_info);
 	}
 
-	return count;
+	return 0;
 }
 
 static void ixgbe_tx_queue(struct ixgbe_adapter *adapter,