mac80211: fix 2.4 GHz 40 MHz disabling

The module parameter ieee80211_disable_40mhz_24ghz
was meant to allow disabling 40 MHz operation in
the 2.4 GHz band by default. However, it is buggy
as implemented because while it advertises to the
AP that the device doesn't support 40 MHz, it will
itself still use 40 MHz configurations.

To fix this, clear the 40 MHz bits from the sband
completely instead of overriding where used.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>