Commit 93c303d2045b3 "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>