mac80211: fix association to 20/40 MHz VHT networks

When a VHT network uses 20 or 40 MHz as per the HT operation
information, the channel center frequency segment 0 field in
the VHT operation information is reserved, so ignore it.

This fixes association with such networks when the AP puts 0
into the field, previously we'd disconnect due to an invalid
channel with the message
wlan0: AP VHT information is invalid, disable VHT

Cc: stable@vger.kernel.org
Fixes: f2d9d270 ("mac80211: support VHT association")
Reported-by: Tim Nelson <tim.l.nelson@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>