ide: unexport ide_set_xfer_rate() (v2)

In cmd64x, siimage and scc_pata drivers:
* don't set drive->init_speed as it should be already
  set by successful execution of ide_set_xfer_rate()
* use hwif->speedproc functions directly

Above changes allows removal of EXPORT_SYMBOL_GPL(ide_set_xfer_rate).

v2:
* updated for scc_pata driver

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