iwlwifi: checking for 40MHz mode

when checking for 40MHz, compare ht_protection to
IEEE80211_HT_OP_MODE_PROTECTION_20MHZ. ht_protection is not a bit-mask
field

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