rt2x00: add driver private field to struct rt2x00_dev

Enhance rt2x00 infrastructure by adding driver specific field to
struct rt2x00_dev.

The new field will be used by rt2800 drivers for chipset registers
access abstraction layer.

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>