iwlwifi: use station HT capabilities and BSS operating mode for Green-field

Green-field mode should be configured in the HT station table.  This patch uses
both the per-station GF support flag as well as the current BSS HT operation
mode (non-GF stations present flag).

Added the "ht_greenfield_support" field to struct iwl_cfg to replace the
device-specific check in rs_use_green().  That check has been moved to
iwlcore_init_ht_hw_capab().

Signed-off-by: Daniel C Halperin <daniel.c.halperin@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>