iwlwifi: generalize iwl4965_send_add_station function

This patch moves iwl4965_send_add_station to iwlcore
under new name iwl_send_add_sta.
Function uses build command handler in order to support
multiple HWs.

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