rt2x00: Introduce concept of driver data in struct rt2x00_dev.

We are getting more and more fields in struct rt2x00_dev that are
specific to one or two of the low-level drivers. Instead of putting
these fields inside the main structure and thus clobbering all low-level
drivers with these fields, introduce the concept of driver data inside
struct rt2x00_dev, whose size is indicated by the low-level driver and
which can be populated by the low-level driver.

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