cmd640: remove cmd_drives[]

* Pass 'ide_drive_t *drive' to check_prefetch(), set_prefetch_mode(),
  program_drive_counts() and cmd640_set_mode().

* Remove no longer needed cmd_drives[].

* Inline setup_device_ptrs() helper in cmd640x_init().

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