net: hip04: mark tx_done() static

We get 1 warning when building kernel with W=1:
drivers/net/ethernet/hisilicon/hip04_eth.c:603:22: warning: no previous prototype for 'tx_done' [-Wmissing-prototypes]

In fact, this function is only used in the file in which it is
declared and don't need a declaration, but can be made static.
so this patch marks this function with 'static'.
Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/hisilicon/hip04_eth.c

@@ -600,7 +600,7 @@ static irqreturn_t hip04_mac_interrupt(int irq, void *dev_id)
 	return IRQ_HANDLED;
 }
 
-enum hrtimer_restart tx_done(struct hrtimer *hrtimer)
+static enum hrtimer_restart tx_done(struct hrtimer *hrtimer)
 {
 	struct hip04_priv *priv;