Staging: quickstart: Use scnprintf in quickstart_pressed_button_show

Use scnprintf instead of snprintf in quickstart_pressed_button_show as
suggested in Documentation/filesystems/sysfs.txt.

Signed-off-by: Szymon Janc <szymon@janc.net.pl>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>