cfg80211: Fix 160 MHz channels with 80+80 and 160 MHz drivers

The VHT supported channel width field is a two bit integer, not a
bitfield. cfg80211_chandef_usable() was interpreting it incorrectly and
ended up rejecting 160 MHz channel width if the driver indicated support
for both 160 and 80+80 MHz channels.

Cc: stable@vger.kernel.org (3.16+)
Fixes: 3d9d1d66 ("nl80211/cfg80211: support VHT channel configuration")
       (however, no real drivers had 160 MHz support it until 3.16)
Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>