mac80211: add missing update on rx status VHT flag

Add missing update on the rx status vht flag of the last
data packet. Otherwise, cfg80211_calculate_bitrate_vht
may not consider the channel width resulting in wrong
calculation of the received bitrate.

Signed-off-by: Chun-Yeow Yeoh <yeohchunyeow@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>