mac80211: protect non-HT BSS when HT TDLS traffic exists

HT TDLS traffic should be protected in a non-HT BSS to avoid
collisions. Therefore, when TDLS peers join/leave, check if
protection is (now) needed and set the ht_operation_mode of
the virtual interface according to the HT capabilities of the
TDLS peer(s).

This works because a non-HT BSS connection never sets (or
otherwise uses) the ht_operation_mode; it just means that
drivers must be aware that this field applies to all HT
traffic for this virtual interface, not just the traffic
within the BSS. Document that.

Signed-off-by: Avri Altman <avri.altman@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>