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: NBen Cahill <ben.m.cahill@intel.com>
Signed-off-by: NZhu Yi <yi.zhu@intel.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>