drivers: ixgbevf: fix unsigned underflow

'count' is unsigned. It is initialized to zero, then it can be increased
multiple times, and finally it is used in such a way:

   >>>> count--;
   |
   |    /* clear timestamp and dma mappings for remaining portion of packet */
   |    while (count >= 0) {
   |            count--;
   |            ...
   ^
If count is zero here (so, it was never increased), we would have a very
long loop :)

Signed-off-by: Kulikov Vasiliy <segooon@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ixgbevf/ixgbevf_main.c

@@ -2935,7 +2935,8 @@ 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;
+	unsigned int offset = 0, size;
+	int count = 0;
 	unsigned int nr_frags = skb_shinfo(skb)->nr_frags;
 	unsigned int f;
 	int i;