iwlwifi: trust mac80211 HT40 setting

When mac80211 asks us to do HT40, it'll not do so
on a channel that we marked as not having HT40+/-
with IEEE80211_CHAN_NO_HT40PLUS (or MINUS). Thus,
there's no need to verify it again in the driver.

Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>