fbcon: replace snprintf in show functions with sysfs_emit

Use sysfs_emit instead of scnprintf or sprintf.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Yang Guang <yang.guang5@zte.com.cn>
Signed-off-by: Helge Deller <deller@gmx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/0cb7ca73d9cd7162988a22a24cd18bbcd3d8bb27.1638156341.git.yang.guang5@zte.com.cn