pata_legacy: fix no device fail path

When pata_legacy can't detect any device, it unregisters the
platform_device and fails detection.  However, it forgets to detach
ata host triggering weird failures as the host later gets freed by
devres while still attached.  Fix it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>