wlcore: fix sparse warning

Use kstrtoul_from_user() for reading the user value,
and fix the following sparse warning:

drivers/net/wireless/ti/wlcore/debugfs.c:937:15: error: incompatible
types in comparison expression (different type sizes)
Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/ti/wlcore/debugfs.c

@@ -929,17 +929,10 @@ static ssize_t beacon_filtering_write(struct file *file,
 {
 	struct wl1271 *wl = file->private_data;
 	struct wl12xx_vif *wlvif;
-	char buf[10];
-	size_t len;
 	unsigned long value;
 	int ret;
 
-	len = min(count, sizeof(buf) - 1);
-	if (copy_from_user(buf, user_buf, len))
-		return -EFAULT;
-	buf[len] = '\0';
-
-	ret = kstrtoul(buf, 0, &value);
+	ret = kstrtoul_from_user(user_buf, count, 0, &value);
 	if (ret < 0) {
 		wl1271_warning("illegal value for beacon_filtering!");
 		return -EINVAL;