iwl3945: store station rate scale information in mac80211 station structure

Currently the rate scale information is pointed to from the mac80211
station structure but since that is the only member we might as well make
it part of the structure. Also move the rate scaling initialization to the
init function, no need to do it when we allocate the structure. This fits
with how mac80211 deals with rate scaling information (it always calls init
after allocation) and makes it easier for us to later call initialization
directly.

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