isci: merge smp request substates into primary state machine

Remove usage of the request substate machine for smp requests identified by:
	task->task_proto == SAS_PROTOCOL_SMP

While merging over the smp_request infrastructure noticed that all the
assign buffer implementations are now equal, so moved it to
scic_sds_general_request_construct.

Reported-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>