stmmac: fix return value check in stmmac_open_ext_timer()

In case of error, the function clk_get() returns ERR_PTR()
and never returns NULL pointer. The NULL test in the error
handling should be replaced with IS_ERR().

dpatch engine is used to auto generated this patch.
(https://github.com/weiyj/dpatch)
Signed-off-by: NWei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

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

@@ -109,7 +109,7 @@ int stmmac_open_ext_timer(struct net_device *dev, struct stmmac_timer *tm)
 {
 	timer_clock = clk_get(NULL, TMU_CHANNEL);
 
-	if (timer_clock == NULL)
+	if (IS_ERR(timer_clock))
 		return -1;
 
 	if (tmu2_register_user(stmmac_timer_handler, (void *)dev) < 0) {