mac80211: fix HT capability overrides for AP station

HT capabilites are asymmetric -- e.g. beamforming is both an
RX and TX capability. If, for example, we support RX but not
TX, the RX capability of the AP station is masked out (if it
supports it). This works correctly if it's really the driver
capability.

If, on the other hand, the reason for not supporting TX BF
is that it was removed by HT capability overrides then the
wrong thing happens: the AP's TX capability will be removed
rather than its RX capability, because the override function
works on own capabilities, not remote ones, and doesn't take
the asymmetry into account.

To fix this make a copy of our own capabilities, apply the
overrides to them (where needed) and then use that to set up
the peer's capabilities.

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