cfg80211: fix missing break in NL8211_CHAN_WIDTH_80P80 case

The switch on chandef->width is missing a break on the
NL8211_CHAN_WIDTH_80P80 case; currently we get a WARN_ON when
center_freq2 is non-zero because of the missing break.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>