cmd64x: fix PIO and MWDMA timings calculations

Just use the standard ide_timing_compute() helper to calculate
PIO and MWDMA timings.  This fixes some issues with the open-coded
version like allowing faster MWDMA timings than the ones required
by the current PIO mode or not accounting for the enhanced MWDMA
cycle time specified by the device.

Based on libata pata_cmd64x host driver.

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>