During pci hot plug, the kernel crashes in a list_add_call

The lookup by tag function will return null if the IOCB is out of range
or does not have the on txcmplq flag set.

Fix: Check for null return from lookup by tag.

Cc: <stable@vger.kernel.org> # 4.12+
Signed-off-by: Dick Kennedy <dick.kennedy@broadcom.com>
Signed-off-by: James Smart <james.smart@broadcom.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>