ipmi: Cleanup oops on initialization failure

Commit 93c303d2 "ipmi_si: Clean up shutdown a bit" didn't
copy the behavior of the cleanup in one spot, it needed to
check for a non-NULL interface before cleaning it up.

Reported-by: Meelis Roos <mroos@linux.ee>
Signed-off-by: Corey Minyard <cminyard@mvista.com>
Tested-by: Meelis Roos <mroos@linux.ee>