serial: max310x: Use devm_clk_get_optional() to get the input clock

Simplify the code which fetches the input clock by using
devm_clk_get_optional(). If no input clock is present
devm_clk_get_optional() will return NULL instead of an error
which matches the behavior of the old code.
Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20201007084635.594991-2-andy.shevchenko@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/max310x.c

@@ -1273,7 +1273,6 @@ static int max310x_probe(struct device *dev, const struct max310x_devtype *devty
 			 struct regmap *regmap, int irq)
 {
 	int i, ret, fmin, fmax, freq, uartclk;
-	struct clk *clk_osc, *clk_xtal;
 	struct max310x_port *s;
 	bool xtal = false;
 
@@ -1287,23 +1286,24 @@ static int max310x_probe(struct device *dev, const struct max310x_devtype *devty
 		return -ENOMEM;
 	}
 
-	clk_osc = devm_clk_get(dev, "osc");
-	clk_xtal = devm_clk_get(dev, "xtal");
-	if (!IS_ERR(clk_osc)) {
-		s->clk = clk_osc;
+	s->clk = devm_clk_get_optional(dev, "osc");
+	if (IS_ERR(s->clk))
+		return PTR_ERR(s->clk);
+	if (s->clk) {
 		fmin = 500000;
 		fmax = 35000000;
-	} else if (!IS_ERR(clk_xtal)) {
-		s->clk = clk_xtal;
-		fmin = 1000000;
-		fmax = 4000000;
-		xtal = true;
-	} else if (PTR_ERR(clk_osc) == -EPROBE_DEFER ||
-		   PTR_ERR(clk_xtal) == -EPROBE_DEFER) {
-		return -EPROBE_DEFER;
 	} else {
-		dev_err(dev, "Cannot get clock\n");
-		return -EINVAL;
+		s->clk = devm_clk_get_optional(dev, "xtal");
+		if (IS_ERR(s->clk))
+			return PTR_ERR(s->clk);
+		if (s->clk) {
+			fmin = 1000000;
+			fmax = 4000000;
+			xtal = true;
+		} else {
+			dev_err(dev, "Cannot get clock\n");
+			return -EINVAL;
+		}
 	}
 
 	ret = clk_prepare_enable(s->clk);