iwlagn: move rate scale initialization to init function

This fits better in how the callbacks operate (alloc does allocation and
init does initialization). This also helps if we later want to do our own
initialization without relying on the mac80211 allocation.

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