panasonic-laptop: use snprintf with PAGE_SIZE in sysfs attributes

Instead of just sprintf() into the page-sized buffer provided
by the sysfs/device_attribute API, we use snprintf with PAGE_SIZE
as an additional safeguard.

Signed-off-by: Martin Lucina <mato@kotelna.sk>
Signed-off-by: Harald Welte <laforge@gnumonks.org>
Signed-off-by: Len Brown <len.brown@intel.com>