IPMI: Add PPC openfirmware unregister

When trying to load the ipmi_si module on a powerpc with no BMC (baseboard
management controller) the driver failes to load correctly, but doesn't
unregister itself from of_platform.  So, on a second modprobe the kernel
crashes.  This patch adds the missing unregister call.

Signed-off-by: Christian Krafft <krafft@de.ibm.com>
Signed-off-by: Corey Minyard <minyard@acm.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>