n is complete, PCS enable bits of unoccupied bits are
turned off unless the controller supports AHCI (ICH6/7 docs mandate
all enables bits are always set on AHCI capable controllers).

Note that PCS present bits are ignored on 6300ESB as described in the
datasheet.  This should fix device detection problems reported with
the controller.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>
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