ipmi: make sure drivers were registered before unregistering them

The ipmi code will never register a PCI or Open Firmware driver if a
hardcoded device is provided by the user by providing device addresses via
the module parameters.  This can cause us to attempt to unregister a
driver that was never registered, resulting in an oops.  Keep track of
registration in order to avoid this.

Fixes a post-2.6.34 regression.

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