cmd640: fix deadlock on error handling

Stop abusing ide_lock lock (switch to a private locking).

Fixes same issue as fixed by Alan Cox in atiixp host driver with
commit 6c5f8cc3.

cmd640 is a bit special cause we still need to leave ide_lock for
->set_pio_mode with 'pio' argument == 8/9 (prefetch disable/enable).

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