mac80211: VHT peer STA caps

Save the AP's VHT capabilities (in managed
mode) and make them available to the driver
in the station information.

Unlike HT capabilities, they aren't restricted
to the common capabilities, so drivers must be
aware of their own capabilities.

Signed-off-by: Mahesh Palivela <maheshp@posedge.com>
[fix endian conversion bug ...]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>