ata: sata_dwc_460ex: remove empty libata callback

The sata_dwc_qc_prep() does nothing.  Use the default ata_noop_qc_prep
instead.

Tested-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: Mans Rullgard <mans@mansr.com>
Signed-off-by: Tejun Heo <tj@kernel.org>