ata: sata_dwc_460ex: get rid of global data

This moves all global data into the driver private struct, thus
permitting multiple devices of this type to be used.

The core_scr_read/write() functions are replaced with equivalent
calls to the existing sata_dwc_scr_read/write().

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