pinctrl: pinconf: fix comparison of different types

Fix the following sparse warning:

drivers/pinctrl/pinconf.c:521:20: error: incompatible types in comparison expression (different type sizes)
Signed-off-by: NJingoo Han <jg1.han@samsung.com>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>

drivers/pinctrl/pinconf.c

@@ -512,7 +512,7 @@ static int pinconf_dbg_config_write(struct file *file,
 	int i;
 
 	/* Get userspace string and assure termination */
-	buf_size = min(count, (sizeof(buf)-1));
+	buf_size = min(count, (size_t)(sizeof(buf)-1));
 	if (copy_from_user(buf, user_buf, buf_size))
 		return -EFAULT;
 	buf[buf_size] = 0;