opti621: program devices timings separately in ->set_pio_mode

* Set drive->drive_data to 'pio + XFER_PIO_0' instead of 'pio',
  then simplify selecting maximum adress setup timing.

* Remove no longer needed compute_pios() and opti621_port_init_devs().

* Program devices timings separately in ->set_pio_mode.

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>