net: hisilicon: fix hip04-xmit never return TX_BUSY

TX_DESC_NUM is 256, in tx_count, the maximum value of
mod(TX_DESC_NUM - 1) is 254, the variable "count" in
the hip04_mac_start_xmit function is never equal to
(TX_DESC_NUM - 1), so hip04_mac_start_xmit never
return NETDEV_TX_BUSY.

tx_count is modified to mod(TX_DESC_NUM) so that
the maximum value of tx_count can reach
(TX_DESC_NUM - 1), then hip04_mac_start_xmit can reurn
NETDEV_TX_BUSY.

Signed-off-by: Jiangfeng Xiao <xiaojiangfeng@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/hisilicon/hip04_eth.c

@@ -248,7 +248,7 @@ struct hip04_priv {
 
 static inline unsigned int tx_count(unsigned int head, unsigned int tail)
 {
-	return (head - tail) % (TX_DESC_NUM - 1);
+	return (head - tail) % TX_DESC_NUM;
 }
 
 static void hip04_config_port(struct net_device *ndev, u32 speed, u32 duplex)