[SCSI] isci: Allow SSP tasks into the task management path.

This commit fixes a driver bug for SSP tasks that require task management
in the target after they complete in the SCU hardware.  The problem was
manifested in the function "isci_task_abort_task", which tests
to see if the sas_task.lldd_task is non-NULL before allowing task
management; this bug would always NULL lldd_task in the SCU I/O completion
path even if target management was required, which would prevent
task / target manangement from happening.

Note that in the case of SATA/STP targets, error recovery is provided by
the libata error handler which is why SATA/STP device recovery worked
correctly even though SSP handling did not.

Signed-off-by: Jeff Skirvin <jeffrey.d.skirvin@intel.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>