iwlwifi: Use sta_bcast_id variable instead of BROADCAST_ID constant

This patch removes iwlYYY_BROADCAST_ID from run time usage.
hw_setting.sta_bcast_id is used instead.

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>