cfg80211: disable 5/10 MHz support for all drivers

Due to nl80211 API breakage, 5/10 MHz support is broken for
all drivers. Fixing it requires adding new API, but that
can't be done as a bugfix commit since that would require
either updating all APIs in the trees needing the bugfix or
cause different kernels to have incompatible API.

Therefore, just disable 5/10 MHz support for all drivers.

Cc: stable@vger.kernel.org [3.12]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>