isci: remove request task context completion state handler

Unlike the other conversions this only updates
scic_sds_io_request_tc_completion() to call the old state handlers directly
(with less verbose names).  This was done for future patch readability, the
implementations have only minor differences for different completion codes.
Without a reference to the function name it would be difficult to dicern which
state is being updated.  Considered changing the order to look up the
completion code before the state but that was not a clean conversion either.

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