ASoC: fsl_ssi: Use the tolower() function

Code can be simplified by using the standard tolower() funtion.
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Timur Tabi <timur@tabi.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/fsl/fsl_ssi.c

@@ -35,6 +35,7 @@
 #include <linux/module.h>
 #include <linux/interrupt.h>
 #include <linux/clk.h>
+#include <linux/ctype.h>
 #include <linux/device.h>
 #include <linux/delay.h>
 #include <linux/slab.h>
@@ -1320,14 +1321,10 @@ static struct snd_ac97_bus_ops fsl_ssi_ac97_ops = {
  */
 static void make_lowercase(char *s)
 {
-	char *p = s;
-	char c;
-
-	while ((c = *p)) {
-		if ((c >= 'A') && (c <= 'Z'))
-			*p = c + ('a' - 'A');
-		p++;
-	}
+	if (!s)
+		return;
+	for (; *s; s++)
+		*s = tolower(*s);
 }
 
 static int fsl_ssi_imx_probe(struct platform_device *pdev,