ide: execute_drive_cmd() cleanup

* Rename 'args' variable in 'if (rq->cmd_type == REQ_TYPE_ATA_TASKFILE)'
  block to 'task'.

* execute_drive_cmd() is used only for REQ_TYPE_ATA_{CMD,TASK,TASKFILE} so
  we can move the common code out from 'if (rq->cmd_type == REQ_TYPE_ATA_CMD)'
  and 'if (rq->cmd_type == REQ_TYPE_ATA_TASK)' blocks.

There should be no functionality changes caused by this patch.

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>