sis5513: sis5513_config_xfer_rate() cleanup

* remove bogus comment for sis5513_config_xfer_rate()
* there is no need to call config_drive_art_rwp() because
  it is called by config_art_rwp_pio()
* remove needless wrapper
* remove stale "TODO" comment
  (IDE core should provide generic tuning code)

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