sc16is7xx: Check for an error when the clock is enabled.

When the clock is enabled, check if there is an error. Otherwise
clk_get_rate() can be called without enabled clock.

Found by Linux Driver Verification project (linuxtesting.org).

Fixes: 0814e8d5da2b ("sc16is7xx: enable the clock")
Signed-off-by: Stefan Potyra <Stefan.Potyra@elektrobit.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/sc16is7xx.c

@@ -1168,7 +1168,10 @@ static int sc16is7xx_probe(struct device *dev,
 		else
 			return PTR_ERR(s->clk);
 	} else {
-		clk_prepare_enable(s->clk);
+		ret = clk_prepare_enable(s->clk);
+		if (ret)
+			return ret;
+
 		freq = clk_get_rate(s->clk);
 	}