mac80211: allow overriding HT STBC capabilities

Allow user to override STBC configuration for Rx and Tx spatial streams.
In practice RX/TX STBC settings can be modified using appropriate
options in wpa_supplicant configuration file:
  tx_stbc=-1..1
  rx_stbc=-1..3

This functionality has been added to wpa_supplicant in commit cdeea70f59d0.

In FullMAC case these STBC options are passed to drivers by cfg80211
connect callback in fields of cfg80211_connect_params structure.
However for mac80211 drivers, e.g. for mac80211_hwsim,
overrides for STBC settings are ignored.

The reason why RX/TX STBC capabilities are not modified for mac80211
drivers is as follows. All drivers need to specify supported HT/VHT
overrides explicitly: see ht_capa_mod_mask and vht_capa_mod_mask fields
of wiphy structure. Only supported overrides will be passed to drivers by
cfg80211_connect and cfg80211_mlme_assoc operations: see bitwise 'AND'
performed by cfg80211_oper_and_ht_capa and cfg80211_oper_and_vht_capa.

This commit adds RX/TX STBC HT capabilities to mac80211_ht_capa_mod_mask,
allowing their modifications, as well as applies requested STBC
modifications in function ieee80211_apply_htcap_overrides.

Signed-off-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

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