HID: wacom - Clean up of sysfs

changed to scnprintf(buf, PAGE_SIZE, ... ) as suggested in sysfs.txt
for show functions

Signed-off-by: Paul A. Tessier <phernost@gmail.com>
Signed-Off-by: Ping Cheng <pingc@wacom.com>
Tested-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>