iwlwifi: move HT_IE_EXT_CHANNEL_* driver definitions to iwl-4965.h

Move HT_IE_EXT_CHANNEL_* driver definitions to iwl-4965.h

Add comments to guide .h file usage (hw/api/driver definitions)

Minor comment cleanup

Signed-off-by: Ben Cahill <ben.m.cahill@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>