isci: cleanup request allocation

Rather than return an error code and update a pointer that was passed by
reference just return the request object directly (or null if allocation
failed).

Signed-off-by: Dan Williams <dan.j.williams@intel.com>

diff --cc drivers/scsi/isci/request.c
Simple merge

diff --cc drivers/scsi/isci/request.h
Simple merge

diff --cc drivers/scsi/isci/task.c
Simple merge