[PATCH] ipmi: fix inconsistent spinlock usage

Part of a patch was accidentally reverted, this corrects an
inconsistent spinlock use in the IPMI message handler.

Signed-off-by: Hironobu Ishii <hishii@soft.fujitsu.com>
Signed-off-by: Corey Minyard <minyard@acm.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>