mac80211: zero center freq segment 2 in VHT oper IE

Clear the Channel Center Frequency Segment 2 in VHT operation
IEs to avoid sending non-zero values if the SKB wasn't zeroed
before adding the VHT operation IE.

Signed-off-by: Chun-Yeow Yeoh <yeohchunyeow@gmail.com>
[change commit message a bit - not necessarily just mesh related]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>