iwlwifi: consolidate station management code

This patch moves code around and group most of the station
management code into iwl-sta.c

No functional changes (yet)

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