opti621: use pre-calculated PIO timings

* Use pre-calculated PIO timings in ->set_pio_mode.

* Remove no longer needed compute_clocks(), cmpt_clk(), struct pio_clocks_s,
  PIO_* defines and OPTI621_DEBUG define.

There should be no functional changes caused by this patch.

Based on a bugreport from Juergen Kosel & inspired by pata_opti.c code.

Tested-by: Juergen Kosel <juergen.kosel@gmx.de>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>