libata: clear saved xfer_mode and ncq_enabled on device detach

libata EH saves xfer_mode and ncq_enabled at start to later set
DUBIOUS_XFER flag if it has changed.  These values need to be cleared
on device detach such that hot device swap doesn't accidentally miss
DUBIOUS_XFER.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>