A domain_device has the same lifetime as its related scsi_target.  The
scsi_target is reference counted based on outstanding commands,
therefore it is safe to assume that if we have a valid sas_task that the
->dev pointer is also valid.

The asd_sas_port of a domain_device has the same lifetime as the driver
so it can also never be NULL as long as the sas_task is valid and the
driver is loaded.

This also cleans up isci_task_complete_for_upper_layer(), renames it to
isci_task_refuse() and notices that the isci_completion_selection
parameter was set to isci_perform_normal_io_completion by all callers.
Signed-off-by: Dan Williams <dan.j.williams@intel.com>