mac80211: fix RX STBC override byte order

The original patch neglected to take byte order conversions
into account, fix that.

Fixes: d9bb4108 ("mac80211: allow overriding HT STBC capabilities")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Reviewed-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>