mac80211: move VHT related RX_FLAG to another variable

ieee80211_rx_status.flags is full. Define a new vht_flag
variable to be able to set more VHT related flags and make
room in flags.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Acked-by: Kalle Valo <kvalo@qca.qualcomm.com> [ath10k]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>