iwlagn: store station rate scale information in mac80211 station structure

Currently mac80211 initializes the rate scaling before notifying the driver
of the station's existence. The driver dealt with this by not relying on
mac80211's station notifications and instead mixing this functionality with
the rate scaling code and other places. To clean this up the driver needs
to do rate scaling initialization after being notified of the station, this
can be done if the rate scaling information forms part of the station
information passed from mac80211 to driver.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>