serial: s5p: Fix clk_get_by_index() error code check

clk_get_by_index() returns negative number on error. Assigning it to
unsigned int makes the subsequent "ret < 0" check always false, leading
in turn to possible unhandled errors. Change 'ret' variable type to
signed int so the code checks and handles clk_get_by_index() return code
properly.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Fixes: cf75cdf96ef2 ("serial: s5p: use clock api to get clock rate")
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>
diff --git a/drivers/serial/serial_s5p.c b/drivers/serial/serial_s5p.c
index 7aeb8c0..fe52580 100644
--- a/drivers/serial/serial_s5p.c
+++ b/drivers/serial/serial_s5p.c
@@ -118,7 +118,7 @@
 
 #if IS_ENABLED(CONFIG_CLK_EXYNOS) || IS_ENABLED(CONFIG_ARCH_APPLE)
 	struct clk clk;
-	u32 ret;
+	int ret;
 
 	ret = clk_get_by_index(dev, 1, &clk);
 	if (ret < 0)