usb: dwc2: get optional clock by devm_clk_get_optional()

When the driver tries to get optional clock, it ignores all errors,
but if only ignores -ENOENT, it will cover some real errors, such as
-EPROBE_DEFER, so use devm_clk_get_optional() to get optional clock.

Cc: Minas Harutyunyan <hminas@synopsys.com>
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Acked-by: Minas Harutyunyan <hminas@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/dwc2/platform.c

@@ -284,10 +284,10 @@ static int dwc2_lowlevel_hw_init(struct dwc2_hsotg *hsotg)
 	}
 
 	/* Clock */
-	hsotg->clk = devm_clk_get(hsotg->dev, "otg");
+	hsotg->clk = devm_clk_get_optional(hsotg->dev, "otg");
 	if (IS_ERR(hsotg->clk)) {
-		hsotg->clk = NULL;
-		dev_dbg(hsotg->dev, "cannot get otg clock\n");
+		dev_err(hsotg->dev, "cannot get otg clock\n");
+		return PTR_ERR(hsotg->clk);
 	}
 
 	/* Regulators */