net: ethernet: dwmac: remove redundant null check before clk_disable_unprepare()

Because clk_prepare_enable() and clk_disable_unprepare() already checked
NULL clock parameter, so the additional checks are unnecessary, just
remove them.
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zhang Changzhong <zhangchangzhong@huawei.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/stmicro/stmmac/dwmac-imx.c

@@ -129,8 +129,7 @@ static void imx_dwmac_exit(struct platform_device *pdev, void *priv)
 {
 	struct imx_priv_data *dwmac = priv;
 
-	if (dwmac->clk_tx)
-		clk_disable_unprepare(dwmac->clk_tx);
+	clk_disable_unprepare(dwmac->clk_tx);
 	clk_disable_unprepare(dwmac->clk_mem);
 }