libata: set device class to NONE if phys_offline

Reset methods don't have access to phys link status for slave links
and may incorrectly indicate device presence causing unnecessary probe
failures for unoccupied links.  This patch clears device class to NONE
during post-reset processing if phys link is offline.

As on/offlineness semantics is strictly defined and used in multiple
places by the core layer, this won't change behavior for drivers which
don't use slave links.

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