mac80211: skip disabled channels in VHT check

The patch "40a11ca8 mac80211: check if channels allow 80 MHz for VHT
probe requests" considered disabled channels as VHT enabled, and
mistakenly sent out probe-requests with the VHT IE.

Signed-off-by: Arik Nemtsov <arikx.nemtsov@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>