mac80211: check if channels allow 80 MHz for VHT probe requests

mac80211: check if channels allow 80 MHz for VHT probe requests

If there are no channels allowing 80 MHz to be used, then the
station isn't really VHT capable even if the driver and device
support it in general. In this case, exclude the VHT capability
IE from probe request frames.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>