mac80211: unset peer's HT 40 MHz support if not usable

When an interface is configured to a 20 MHz channel
and the device as well as the peer are 40 MHz capable
the HT capabilities of the peer are not restricted to
20 MHz, even though they're supposed to be restricted
to the currently possible capabilities.

Unset the 40 MHz HT capability bits in this case.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>