IPMI: fix hotmod remove lock

The removal of proc entries was done holding a lock, which is no longer
allowed.  There is no need for the lock, only a mutex is required, so switch
over to a mutex.

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>