ixgbevf: Fix signed/unsigned int error

In the Tx mapping function if a DMA error occurred then the unwind of
previously mapped sections would improperly check an unsigned int if
it was less than zero.  Changed the index variable to signed to avoid
the error.
Signed-off-by: NGreg Rose <gregory.v.rose@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ixgbevf/ixgbevf_main.c

@@ -2943,9 +2943,10 @@ static int ixgbevf_tx_map(struct ixgbevf_adapter *adapter,
 	struct ixgbevf_tx_buffer *tx_buffer_info;
 	unsigned int len;
 	unsigned int total = skb->len;
-	unsigned int offset = 0, size, count = 0, i;
+	unsigned int offset = 0, size, count = 0;
 	unsigned int nr_frags = skb_shinfo(skb)->nr_frags;
 	unsigned int f;
+	int i;
 
 	i = tx_ring->next_to_use;