ipmi: simplify locking

Now that the the IPMI driver is using a tasklet, we can simplify the
locking in the driver and get rid of the message lock.

Signed-off-by: Corey Minyard <cminyard@mvista.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>