isci: save the i/o tag outside the scic request structure.

The pointer to the core representation of a request is marked NULL at
completion, but we need to save the i/o tag for task management.

Signed-off-by: Jeff Skirvin <jeffrey.d.skirvin@intel.com>
Signed-off-by: Jacek Danecki <Jacek.Danecki@intel.com>
[revise changelog]
Signed-off-by: Dan Williams <dan.j.williams@intel.com>