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/ixgbe/ixgbe_main.c b/drivers/net/ixgbe/ixgbe_main.c
index 9c9202f..6d61add 100644
--- a/drivers/net/ixgbe/ixgbe_main.c
+++ b/drivers/net/ixgbe/ixgbe_main.c
@@ -5167,14 +5167,14 @@
 	tx_buffer_info->dma = 0;
 	tx_buffer_info->time_stamp = 0;
 	tx_buffer_info->next_to_watch = 0;
-	count--;
+	if (count)
+		count--;
 
 	/* clear timestamp and dma mappings for remaining portion of packet */
-	while (count >= 0) {
-		count--;
-		i--;
-		if (i < 0)
+	while (count--) {
+		if (i==0)
 			i += tx_ring->count;
+		i--;
 		tx_buffer_info = &tx_ring->tx_buffer_info[i];
 		ixgbe_unmap_and_free_tx_resource(adapter, tx_buffer_info);
 	}