iwlagn: Remove ht40 support from 5.2GHz for _bgn devices

For _bgn device, remove ht40 support for 5.2GHz, it is probably ok since
the "band" is not support but just feel strange.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>