iwlwifi: unify iwl_setup_rxon_timing

This patch unifies setup_rxon_timing funcions
of AGN and 3945. HWs differ only in supported maximal
beacon interval. This is reflected in hw_paras.max_beacon_itrvl

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>