scc_pata: PIO fixes

* Use pio == 255 == "auto-tune" in scc_config_drive_for_dma() instead of
  forcing PIO4 on PIO fallback.  Fix comment while at it.

* Rename scc_tuneproc() to scc_tune_pio() and add scc_tuneproc() wrapper.
  Move finding of the best PIO mode and setting of transfer mode on the device
  to the new wrapper.

* Fix scc_tune_chipset() to tune PIO modes.  Do a small cleanup while at it.

Acked-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>