e1000/e1000e/igb/igbvf/ixgb/ixgbe: Fix tests of unsigned in *_tx_map()

The variable count and i are unsigned so the (<|>=)0 tests do not work.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/ixgb/ixgb_main.c b/drivers/net/ixgb/ixgb_main.c
index bcd0f01..593d1a4 100644
--- a/drivers/net/ixgb/ixgb_main.c
+++ b/drivers/net/ixgb/ixgb_main.c
@@ -1363,13 +1363,13 @@
 dma_error:
 	dev_err(&pdev->dev, "TX DMA map failed\n");
 	buffer_info->dma = 0;
-	count--;
-
-	while (count >= 0) {
+	if (count)
 		count--;
-		i--;
-		if (i < 0)
+
+	while (count--) {
+		if (i==0)
 			i += tx_ring->count;
+		i--;
 		buffer_info = &tx_ring->buffer_info[i];
 		ixgb_unmap_and_free_tx_resource(adapter, buffer_info);
 	}