cfg80211: module_param to disable HT40 in 2.4GHz band

Currently mac80211 uses ieee80211_disable_40mhz_24ghz module
parameter to allow disabling 40MHz operation in the 2.4GHz band.
Move this handling from mac80211 to cfg80211 so that the feature
will be more generic.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>