iwlwifi: configure HT40 channels in iwl_mac_config

As indicated by note in iwl_ht_conf, some HT parameters are set on association
(e.g., channel width) and some vary over time (HT protection mode) and per
station (e.g., short GI support).  The global parameters should be set in
iwl_mac_config and the local/varying parameters in iwl_ht_conf.

This patch moves the channel width configuration from iwl_ht_conf to
iwl_mac_config, and defers further cleanup of the local/global conflation for a
later patch.

This fixes a bug in using HT40 channels in some modes.

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>