video: fbdev: udlfb: replace snprintf in show functions with sysfs_emit

Use sysfs_emit instead of scnprintf, snprintf or sprintf.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Jing Yao <yao.jing2@zte.com.cn>
Signed-off-by: Helge Deller <deller@gmx.de>