hwmon: (ltc2947) Properly handle errors when looking for the external clock

The return value of devm_clk_get should in general be propagated to
upper layer. In this case the clk is optional, use the appropriate
wrapper instead of interpreting all errors as "The optional clk is not
available".
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20210923201113.398932-1-u.kleine-koenig@pengutronix.deSigned-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/ltc2947-core.c

@@ -989,8 +989,12 @@ static int ltc2947_setup(struct ltc2947_data *st)
 		return ret;
 
 	/* check external clock presence */
-	extclk = devm_clk_get(st->dev, NULL);
-	if (!IS_ERR(extclk)) {
+	extclk = devm_clk_get_optional(st->dev, NULL);
+	if (IS_ERR(extclk))
+		return dev_err_probe(st->dev, PTR_ERR(extclk),
+				     "Failed to get external clock\n");
+
+	if (extclk) {
 		unsigned long rate_hz;
 		u8 pre = 0, div, tbctl;
 		u64 aux;