mac80211: add HE 6 GHz Band Capability element

Construct HE 6 GHz band capability element (IEEE 802.11ax/D6.0,
9.4.2.261) for association request and mesh beacon. The 6 GHz
capability information is passed by driver through iftypes caps.

Signed-off-by: Rajkumar Manoharan <rmanohar@codeaurora.org>
Link: https://lore.kernel.org/r/1589399105-25472-7-git-send-email-rmanohar@codeaurora.org
[handle SMPS, adjust for previous patches, reserve SKB space properly,
 change to handle SKB directly]
Link: https://lore.kernel.org/r/20200528213443.643aa8101111.I3f9747c1147480f65445f13eda5c4a5ed4e86757@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>