ipmi: Fix NULL pointer access and double free panic.

ipmi: Fix NULL pointer access and double free panic.

Orabug: 24697944

In case of a ldom/hardware not supporting ldc, ipmi_si module
will set the smi interface pointer to NULL after ldc channel
detection failure. However, ipmi_si module will crash during
unload due to absence of NULL check.

Add the smi interface null check and assign the workqueue to
NULL during cleanup to avoid double free panic.

Signed-off-by: Atish Patra <atish.patra@oracle.com>
Reviewed-by: David Aldridge <david.j.aldridge@oracle.com>
Reviewed-by: Vijay Kumar <vijay.ac.kumar@oracle.com>
Reviewed-by: Rob Gardner <rob.gardner@oracle.com>
(cherry picked from commit f2546771efb0c6402a5ea65dac9c5dbce18150e6)
Signed-off-by: Allen Pais <allen.pais@oracle.com>