Sanitize 66MHz clock use: "enable" 66MHz clock before starting UDMA3/4/5
read/write transfer and "disable" it after finishing transfer.

- fixes timings for non-UDMA3/4/5 operations (correct 33MHz timings are used)

- allows using UDMA3/4/5 modes on a capable drive even if non-UDMA3/4/5 drive
  is present on the same channel

- fixes corner case when one drive on the channel was using UDMA66/100 + LBA48
  (so clock was enabled/disabled for each read/write) and other one was using
  UDMA66/100 + LBA28, it could happen that request on LBA48 drive disabled
  66MHz clock and it was not enabled for the next transfer on LBA28 drive