sata_nv: reinstate nv_hardreset() for non generic controllers

Commit 2fd673ec

 which tried to remove
hardreset for generic accidentally removed it for all flavors as all
others were inheriting from nv_generic_ops.  This patch reinstates
nv_hardreset() and puts it into nv_common_ops which all flavors
inherit from.  nv_generic_ops now inherits from nv_common_ops and
overrides .hardreset to ATA_OP_NULL.

While at it, explain why nv_hardreset and ATA_OP_NULL override are
necessary.

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