ide-pm: don't abuse rq->data

Impact: cleanup rq->data usage

ide-pm uses rq->data to carry pointer to struct request_pm_state
through request queue and rq->special is used to carray pointer to
local struct ide_cmd, which isn't necessary.  Use rq->special for
request_pm_state instead and use local ide_cmd in
ide_start_power_step().

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>