[PATCH] I8K: fix /proc reporting of blank service tags

Make /proc/i8k display '?' when service tag is blank in BIOS.
This fixes segfault in i8k gkrellm plugin.

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>