mac80211: fix HT beacon-based channel switch handling

When an HT AP is advertising channel switch in a beacon, it
doesn't (and shouldn't, according to 802.11-2012 Table 8-20)
include a secondary channel offset element. The only possible
interpretation is that the previous secondary channel offset
remains valid, so use that when switching channel based only
on beacon information.

VHT requires the Wide Bandwidth Channel Switch subelement to
be present in the Channel Switch Wrapper element, so the code
for that is probably ok (see 802.11ac Draft 4, 8.4.2.165.)

Reported-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>