net: stmmac: ensure jumbo_frm error return is correctly checked for -ve value

The current comparison of entry < 0 will never be true since entry is an
unsigned integer. Make entry an int to ensure -ve error return values
from the call to jumbo_frm are correctly being caught.

Detected by CoverityScan, CID#1238760 ("Macro compares unsigned to 0")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -2947,7 +2947,8 @@ static netdev_tx_t stmmac_xmit(struct sk_buff *skb, struct net_device *dev)
 	int i, csum_insertion = 0, is_jumbo = 0;
 	u32 queue = skb_get_queue_mapping(skb);
 	int nfrags = skb_shinfo(skb)->nr_frags;
-	unsigned int entry, first_entry;
+	int entry;
+	unsigned int first_entry;
 	struct dma_desc *desc, *first;
 	struct stmmac_tx_queue *tx_q;
 	unsigned int enh_desc;