isci: unify request start handlers

Unify the implementation in scic_sds_request_start and kill the state
handler.

Reported-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Piotr Sawicki <piotr.sawicki@intel.com>
[remove scic_sds_request_constructed_state_start_handler]
Signed-off-by: Dan Williams <dan.j.williams@intel.com>