iwlagn: remove common station priv

Since the driver split there's no more need for
shared/non-shared private station data so remove
struct iwl_station_priv_common entirely.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>