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: NChristian Lamparter <chunkeey@googlemail.com>
Signed-off-by: NMans Rullgard <mans@mansr.com>
Signed-off-by: NTejun Heo <tj@kernel.org>